I can't go for a brand new registration unless everything is new (you are allowed to use one reconditioned component) but I'll be happy enough with the original number from the donor car or an age-related 1991 J Reg.

I only bought the shell because it had a receipt, and DVLA have had that and were happy to issue a new VIN number - from the factory, service shells have no VIN of their own. I also have the original consignment plate that rivets on where the VIN plate would normally be.