The big drawback is that of the era.  The problems of technology means that the Keeper would need to do a lot of research and revision to maintain the course of clues and discovery while allowing for the new ways in which information can be developed.  The political changes would also require a great deal of research.  For instance, the Cold War has come and gone in the 73 years since MASK's original setting, and it influenced many of the regions which MASKS features.

The resources available to DG investigators in international investigations will certainly be different from the resources they have within the US or its territories, but I wouldn't say that that removes the distinctive flavor of Delta Green; it simply changes the focus a bit and makes things more challenging for the players and the Keeper.  More challenging for the players because they need to bear in mind that their resources are less comprehensive, and more challenging for the Keeper because you'll need to get very familiar with exactly what those resources will be and some of the consequences of using them. 


Government agents abroad
Basically, DG operating in another country will make it a black operation, even more so than a DG operation within the US.  The CIA and the State Department (for disinformation--or for an obstruction if the players don't have the right influence) are likely to be the primary agencies, rather than the FBI and the other federal law enforcement agencies which are typically featured most in domestic DG operations.  At the core things aren't that different:  in both domestic and international operations the players must cover up everything they've been doing and invent some legitimate pretense so nobody looks to closely at why they have been doing what DG does. 

But the methods of covering it up and the consequences of exposure will certainly be different.  The CIA has conducted black operations in other countries throughout its history, the most bungled and notorious of which have made headlines and soured US relations with various other governments at different points.  So the players will have recourse to agents who are used to conducting coverups and disinformation... but not always with complete success. 

So if it comes out that the players have, say, applied a few hundred pounds of plastique to the home of a prominent Belgian businessman with alleged ties to a mysterious Swiss apocalypse cult, they can experience all manner of diplomatic and legal problems.  They might have been operating under diplomatic cover, with papers issued by the State Department declaring that they are not to be interfered with (intelligence officers and State Department security officers have at time used the "diplomatic pouch" to transport weapons or other items not officially allowed), in which case they will (if the local police are good enough to respect the treaties and conventions) behanded to the US government and declared persona non grata, probably with a request that they be prosecuted at home for their violations.  If they are not lucky enough to have diplomatic cover, then they can expect to be arrested and tried locally, the same as any law breaker.  Diplomatic pressure at this point might result in the local government agreeing to drop the charges and return the offenders to the US government, but for more egregious allegations this will be much less likely.  For DG, influence "behind the scenes" might be more likely, with US intelligence officers negotiating with local police, intelligence, or politicians to arrange the agents' release under a cover of "diplomatic" cooperation.

Delta Green agents won't be in the same position as regular Call of Cthulhu investigators; on the other hand, they will certainly have an all new set of challenges.  State Department personnel are notorious for their antipathy for CIA and other black operations, for instance; and DG agents in a bind might find that the local ambassador or senior Foreign Service Officer has no desire whatsoever to cooperate when the CIA station chief asks for help extricating a group of agents who were in country illegally and caught doing illegal things.  And I haven't even touched on the possibility that MJ-12 will have its own influence in the State Department and its own observers out there watching for DG-type occurances and allegations and "suspects" (i.e., the players!)  to debrief.

DG operatives in foreign countries makes the characters and keepers jobs more difficult. Naturally all difficulties can be overcome, but the fact that they are Americans operating abroad does change the balance of power. After all, the primary job of the CIA (and NSA) is intelligence gathering. Admittedly assasinations, election rigging and assorted dirty tricks comes into it, but overall foreign operatives are supposedly secret operatives and shouldnt be doing things that draw attention to themselves, like killing prominent Britons, or Egyptian businessmen, or blowing up Pyramids and conducting naval assaults in the South China Sea. In short, the very negative consequences of being caught are highly likely with this campaign. And DG, isnt really going to give them a get out of jail free card, after all they've got the worlds media attention on them (Just imagine what Egypt or China would do if they caught some CIA agents!! How many million documentaries would CNN or the BBC make? How many billions in sanctions would have to be lifted to get them back?), and attention is the last thing DG wants. Ever. Even backroom deals would gather attention from MJ-12, any publicity, which would remain with for the rest of their careers, and  so it is sanitisation time.

A second problem factor arises from the fact that not only do they have the normal problems traditional investigators have, but they also have the host countries counter-espionage agencies to deal with. There is naturally a chance that the players will be caught for merely entering the country. In short, its a lot more bloody and dangerous for DG operatives overseas, and is especially so in the context of this extended campaign.

But it is possible to use American DG's abroad, but the Keeper has to change the structure somewhat. How about assuming that DG has a greater than published degree of influence within the arms of foreign relations and intelligence. Then you could use teams of specialists attacking the problem from all angles. This would mean each player would have multiple characters instead of one (or lots in a chronological sequence as is traditional with Cthulhu). That way, like in Escape from Innsmouth, you would have say FBI and NATO liasons in London, then switch to a CIA embassy chief in Cairo and his established contacts and team, and then use a variety of DEA agents, or commercial attaches' and spies in the Pacific. Not only does it do away with explaining why the London desk is stepping on toes by sending agents to Australia  but this is also an elegant way of dealing with Masks' mortality rate.  If your dead, well you can try again in the next country. And remember- "You were only supposed to blow the bloody doors off."


Satellite Imaging
Both CTHULHU NOW and DELTA GREEN looked a little at this issue, and there are a few possibilities of greater or lesser interest and plausibility. DG snags and classifies IMGINT before it can warp sanity. Ryleh and Leng aren't sufficiently within this dimension to be detected by our current sensing systems. IREM has been discovered, but the Saudi Arabian Govt. won't authorize any scientific teams, and they WILL rebury it if it get uncovered. The idea that seems to fit best with Delta Green stems from the fact that US satellite imaging is handled largely by the National Reconnaisance Office, which is largely under the purview of MJ-12.  So it's possible that satellite reconnaisance has picked up plenty of files on those big alien ruins in Antarctica, and it's entirely likely that various navies and even private or academic organizations have observed certain anomalies in deep fissures of the South Pacific; and certainly the US Navy has had a run-in or three with the cities of the Deep Ones.  If the Keeper takes these to be the facts, then one needs a story to explain how these astounding facts could still be secret or ignored by the world at large:  conspiracy and disinformation.  Maybe there is a worldwide conspiracy of government and private individuals, of which MJ-12 is a (dominant, given the Accord and the Report) partner.  In this case, satellites and oceanographers have indeed picked up R'lyeh and the city of the Old Ones, but these secrets have been closely held by this ruling cabal and kept from subsequent discovery and notice by vigilant disinformation and distraction.


Occult instead of government
Convert Masks to DG is maybe best done as an occult campaign rather than an anti-MJ12 one. The Keeper also has to decide if he wants to make it 'anti-Fate'.  It makes sense as not only does the campaign kick off in NYC anyway but it also involves Nyarlothotep. And we all know what they say about Alzis, eh? Therfore, the Fate can replace the Bloody Tongue, or possibly just give them a safe house in NYC. Given this, ignore the Greys, their masters and their allies entirely. Their brand of sci-fi alien invasion would distract from the occult conspiracy slowly unfolding before the characters.

As for updating the Carlyle Expedition, the modern surveillance techniques are going to be of little use in locating them, or their former path.(Credit cards traces in Kenya? I think not.) I would certainly make Sir Aubrey and the Australian chappie Fate members, but would probably keep the Kenyan woman as belonging to the Bloody Tongue. However, the power of the Fate is going to be severly restricted outside of the US anyway, so the Keeper can probably keep most of the original cults. (e.g. Black Pharoah and Bloated Woman are fine, and the Penhew Foundation could easily still exist).

Masks is bloody lethal anyway, the addition of a bunch of trained, well resourced magical assassins will only increase the body count tenfold, therefore the Keeper is equally justified in only giving the Fate a marginal role. However an Idea role when the PC's meet to Black Pharoah to spot a distant resemblence to Alzis might be nice. (And it would be even more confusing if Alzis were to help them out as well. Are they warring twins? Does Nyarlothotep have Multiple Personality Disorder (MPD)? Or are they merely involved in some incalcuble, and inscrutable plot of the most fiendishly cunning God in the universe? Or even, All of the above!!)

     
A few ideas

New York: Bloody Toungue Street Gang working under the Fate. Led by the Lord of Blood

London: Encounters with MI-7 and British Intelligence. Werewolf could be a MJ-X biologic project, or yet Another WW/Storyteller crossover. Cult could be stationed in the Severn Valley.

Kenya: Tribal conflict, refugees, Ebola scares, AIDS Highway and a connection to the Fate. Go read the "HOT ZONE" by Richard Preston.

Cairo: Ever see the James Bond flick where 007 and Jaws duke it out near the Sphinx? Tourists get in the way here! Middle East terrorists. Or set it in Iraq: Zoroastrian ziggurats, Russian Intelligence, Saddam Hussein, Kurdish Kultists and the Heroic United Nations.

Shanghai: Set it in Hong Kong: Tcho-Tcho, triads, Jackie Chan Flix, the Red Chinese, and a secret stealth ICBM or maybe a MJ-X UFO.