I agree, when so much will be replaced, how can you call it in "original" condition!
It is like my other passion; steam railway engines. When they are restored, especially from ex-Barry condition, so many parts are new replacements. But, as long as the frames are "original" then the whole engine is considered to be the same. So many "original" parts can also come from other, long expired, members of that class of loco's. That also justifies the "original" badge. But if the frames are replaced, along with having a new boiler, then it cannot possibly be classed as "original".
The same must apply to motor vehicles. If the body shell is brand new, or one extensively rebuilt, with an engine that never belonged to that vehicle, then, I think, it cannot be "original".