What he said!
Coach said it all. It basically breaks down like this. With the right license (which you do not have or you would already know the answer to your Q) you can restore a de-militarized full auto weapon or reconvert to full auto a weapon that was modified to fire only as semi-auto (using the original receiver), but there are a lot of restrictions and gotcha's. What you cannot do is remanufacture a new or used semi-auto weapon to fire full auto or build a new full auto weapon for private use.
Example: if you had a vintage Thompson receiver even if cut into pieces(!) you could weld it back together and add all the parts needed to build a Model 1928 (or whatever). You could even take parts from two receivers and weld the pieces back into one receiver. All this assuming you have the correct license and have filed all the paperwork. BATF says the receiver is the GUN.
You cannot buy a parts kit and legally make a new full auto receiver, again since the receiver is the GUN.
I am not a lawyer and this is only my understanding of the way things are laid out from my own reading on the subject. Before I ever undertook such a project I would want to be in correspondence with someone that does this for a living, a lawyer that specializes in such issues and the BATF.
Anyone that still wants to do this, without a lot more research is taking a huge chance.