the "shell" of the car, ie the chassis and passenger compartment or whatever everything bolts onto is what has the ID

this ID is unique to that shell

you can change everything else that's either bolted, riveted or welded to that shell but as long as that originality is retained there is no question of breaking the law

so you can put a new roof on, or a new floorpan, or new chassis legs or a pillars or b pillars or c pillars etc as individual items but changing the whole lot would be breaking the law

unless

you have a new unregistered shell, in which case you can legally reshell the old car to the new shell and then it gets a q plate

however

if the parts that you are fitting to this "new unregistered shell" are "new, bought from the manufacturer and proven to be new with paperwork" then, once you have obtained enough "new" parts you can get an age related plate to go with the car

i would hazard a guess that with so much hard work the car either has exceptional sentimental value or extreme monetary value, either way, doing it wrong and then bumping into the wrong copper will see it crushed, worst still if you were to have an accident and these things came to light