What you can't do is pick and choose from different catalogues, years and classes, in that regard nzeder is absolutely correct. This of course is easy with popular race cars such as Escort, Capri, BMW '02 etc especially if the homologation papers are available.

You raise an interesting question regarding British Leyland Comps dept. Around 1968 BL Competitions decided to build a Group 2 Rover P6B project car. They built at least one and possibly more and Roy Pierpoint drove the car in some club events and at least one European long distance event where the thing blitzed the field before retiring. In typical BL manner the whole exercise was half arsed, and rife with in-fighting, the car and modifications were never homologated and the Comps dept closed down shortly afterwards, I understand the car went to Aus.

Today there appear to be a number of replica/ reproductions ( even assuming the original car(s) survived) racing in the UK and Europe where the need for documentation is far stricter than here. This raises the question of what is the process involved in ratifying such a car because it seems this situation is no different to say the Custaxie2 or possibly Ray's Marcos.