On the forms, is Stemple listed as the "manufacturer"? If so, then technically he should have done something to the receiver to "manufacture" it into an NFA weapon other than just the registration paperwork. If the side of the weapon says its an M16A1 by Hydramatic/GM, it should be a reweld since it definitely started life as a machinegun and wasn't converted from a semi by Stemple. That said, I don't think Stemple was especially well known for his welding ability (like John Norrell is). Look very carefully - the weld seams should be visible by careful inspection. Check the inside of the mag well and interior spaces of the receiver for evidence of rewelding. It may be that Stemple somehow came across this receiver (uncut) and just registered it without actually doing anything to it.
Value? I'd fathom a guess of $10k-15k (depending upon if its really a reweld or whether it was never cut and just registered by Stemple). As to building it up - the original parts are gone, so its not as if you have a NIB weapon. Build it up, shoot and enjoy it.
2) What is it worth? 3) Is it worth more as a collector piece not built up? 
Thanks for any help, Jamie