Can the receiver be replaced and the registry entry adjusted to reflect the new serial number?
Legally? Absolutely not. I have to believe that your wonderful legislators who passed the '86 ban intended on all of the old machineguns eventually failing beyond repair. That was the intent of the legislation, wasn't it?
You can fix it, but you cannot replace the receiver. Ruger was replacing broken AC556 receivers (rare) - my understanding is that they were replacing them with the few pre-86 receivers they still had in stock.