"replacement parts" are not nessesarily regulated as are the recievers.
For instance you can own all manor of AR full auto parts,
Same goes for HK, Cetme, AK........
and so on, and not be in violation of any laws.
The danger lies when you have both the arms and the fun parts.
Even then, some will say you are ok as long as you have not modified, nor fit any parts to your particular weapon.
Others will have palpitations at the thought....
There is a danger.....but to what extent......I don't know.
The ATF seems to like to keep things in a grey area.
It makes it easier for them to "operate".
Perhaps up at the Machinegun Board they may have some info to ease your worries, or not.