Are there limitations in US legislation on a US citizen to own a MiG-15 in Europe? If US citizen buys cold war era MiG-15 jet fighter aircraft, which:
- had been acquired from the museum (facility of the Ministry of Defense one of the European Union countries, member of NATO) by private person (contract document is available);
- is still located in that country;
- at least for time being the plane will stay located in that country even after the US citizen buys it from that private person;

Now, since that US citizen is also dual citizen of that country:
- Is there anything which he needs to do to be in compliance with US law?
- Is there anything which he should not do to be in compliance with US law?
- Or, since the plane will stay in that country (no import/export will take place), is everything basically governed by that country law only and he is good just to stay in compliance with the law of that country?

Does the same apply to any weapon/armament accessories present like cannons, gun camera, gun sight, ejection seat? E.g. cannons are now independently stored at the third party, licensed to handle military material. He would not get cannons to his possession until unless they are modified (deactivated) according to the law of that country.
With mere possession the only issue you might run into is any live, functional weapons on the aircraft. This is governed more by the nature of the device than the person owning it. Generally speaking, Europe is far less accommodating regarding private citizen ownership of military weapons than the US, especially nasty ones like automatic cannons. If they have been demilled to the specifications of the country in which they reside, then they're not guns any more, just decorations. 

A couple of years ago, I saw a Soviet era MI-24 attack helicopter for sale in the US. Main and tail rotors present, no engines, no weapons, cockpit was pretty much gutted, airframe was past its useful life, for static display only. It looked intact from the outside... and also looked very sinister. For some odd reason, my wife balked at me spending $8k on this ultimate lawn decoration. But, there were no regulations regarding its ownership, other than maybe a neighborhood association getting huffy about someone parking an attack helicopter in their front yard.  

Now, if you want to actually fly the MIG15, that's a different matter. But if you as a US citizen want to purchase a MIG15 as a static display, without functional weapons, the only thing the US government is going to care about is whether you paid their income tax on the money you use to buy it.