If the wrecking yard compressor has the same seal type at the manifold, then I personally would give it a try.

That said, I have a recovery/recycle machine, so if I have to abandon it, I can recycle the R12. If you don't have a means of reclaiming the refrigerant, then it's a crap shoot. If the compressor feels good when turned, smooth and you can feel the compression then the only thing you can't test is the shaft seal.

Was the system you took the compressor off of still holding a charge? If so that would tell you that the shaft seal is probably okay.

For the money you're saving trying this compressor, you can afford to lose R12 although it is unlawful to charge a system with a known leak. In your case you don't know that you have a leak or not.

Good luck