First, where's the plane? If it's still in the A&P's possession, odds are he can just hold it under state mechanic's lien laws until paid, and if not paid within some time specified in the law, dispose of it without further permission from the owner, keeping the money owed (plus expenses) and returning the balance to the owner (or in this case, the owner's estate).
If not still in the A&P's possession, then was the aircraft actually deregistered, or did the registration just expire? If just expired, then he can file a lien with the FAA even though the aircraft does not have a current registration. To actually deregister an aircraft, you have to go through a special process with the FAA, but unless documentation was filed to change the ownership or it was reported destroyed or exported, it's probably still listed as owned by the last owner and a lien can be filed against it even though it can't legally be flown. You can check on this with the last N-number at the FAA Registry web site. Beyond that, it would be a matter of state law, and the A&P will need a local attorney to help sort it out.