OK,so as suspected, the piston rule is not pertinant to the conversation. HOWEVER:

I agree with Joe in a practical sense, by probably not in application. Here is why...A forged widget of identical exterior dimensions to a cast widget should weigh less by nature...making it illegal to the replacement part rule.

IF you could match the stock part in every dimension - and weight is included in that, then I agree with Joe that the method of manufacture is not dictated by the rules...but are there really dimensionally identical parts out there that are forged that weigh the same as cast (stock)?