This actually happened, Guy I know back in the 80`s, bought 2 indentical old(er) cars. He ran one against the other, when one was due MOT or repair, he merely swapped the plates. With only one on the road at any one time the vin plates where identicaly stamped, the engine he left, the argument being he could get away with having swapped engines, just forgotten to register change to DVLA, as many old cars had. We had swapped engines regularly on Rally Cars! we never told DVLA because next week we could be doing it again!, "borrowing" drive trains was a regular occurance.
Pete.