Should just be the main part of the receiver that you have to trash. Just cut it off and leave it in the trash before you return. The barrel shroud you sould be fine. If in doubt, contact the ATF directly.
Everything else you can import. Technically, if its not a barrel, bolt or receiver and the value is less then $100 you can import it without a permit. But, its always easier to just put one in anyways. If you are an American GI in Germany (I am one in Stuttgart), bring stuff back is not a problem.
The sad part is, the parts are not really worth that much here in the US. A deko MG3/42/52 starts at about 500 Euros ($600). Dont think the sum of the parts in the US would be that much, especially with a welded over barrel and bolt.
If you want to buy one as a wall hanger, do it. But dont expect to come out a head with the parts. You used to be able to get over with the high end stuff- Steyr Aug, Sig 55x, etc but the exchange rate is horrible right now.
If you need any other info, contact me offline.