Was pondering this the other day, as you do.
If I was to buy a CB750 frame, for example....get it properly UK registered. and then build a bike around it from a more modern vehicle. Lets say, a 900 Hornet or late model CB750 DOHC.
UK registration is based around the frame number from what I recall, so I guess it could legally stay registered with it's 1970's numberplate, and be considered a 1970's vehicle. But as you swap the engine, suspension, wheels, brakes ... does it still get considered as a 1970's vehicle in terms of these things? Is there a point at which your vehicle ceases to be a tax, MOT and emissions exempt vehicle and becomes something else. Or does it just stay as a 'modified' classic vehicle.
I know it would involve an interesting conversation to insure it .... but thats another question.