the bits that are restricted as replica firearms are the receivers of the guns, usually the piece that has the serial number on the real steel counterpart, but in some cases they may say any major component of the gun body/receiver is inadmissible.
just your guns would be restricted if they don't meet the minimum requirement. It's fucked up, but thems the rules. Anything under 366fps with a .2 is a replica firearm.. replica devices are prohibited to own by federal law. The whole reason we can bring >366 in is by a loophole that could be closed at any time.
Parts are easy to get, they are not restricted in any way, so you could order parts from anywhere, barring that they are not pistol frames, upper or lower receivers. There have been some cases of complete gearboxes being stopped at the border as well but I think in most cases they are allowed to pass.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.