If you build entirely of new bits then it should be registered as a new vehicle and given a related plate, HOWEVER
If you nut and bolt rebuild a landy you can legitimately use the original plates as long as you have kept it original!
So you can buy a new chassis, bulkhead, engine etc and as long as they were as originally fitted then no probs.
The purists would not call it a rebuild, but in the eyes of the law you would be OK.