Suppose it depends on the aircraft. I'm guessing here and please, those who know better should jump in and correct me, but for instance, the Hurricane had a lot of fabric covering on the fuselage and control surfaces, after 60 years one would expect that to have been replaced at least once! Guess the same applies to the Gladiator flying out of Shuttleworth. Other aircraft may well have original skins and many original parts. I know that modern military aircraft have a manufactureres plate, rather like a VIN plate that you would find on a car. To me at least, that is the aircraft. It is IMO really irrelevant how much of the rest has been changed, as long as it can be traced. Bit like a sweepers broom really. In all honesty does it really matter as long as artefacts like these aircraft can be kept in a safe airworthy manner to return to their natural element?