There is an interesting historic Mini which is still around.
The owner bought an 850 in 1961 and started club rallying in it. He then bought a 997 engine & gearbox together with 7" disc brakes and it became a 997 Cooper for competition purposes. That was in around 1963.
Later it acquired a real 1275 Cooper 'S' engine and brakes and it did a lot of international rallies, always entered as a Cooper 'S'. The log book had it as a Mini with the engine size as 1275 cc.
I think it had a re-shell into a second-hand 850 shell in around 1970 all done with the help of a BMC/BLMC main dealer who used to help with entry fees and parts.
Now the real 'purists' will decry this as a 'fake', but we can all make up our own minds about that I guess.
I won't name the owner, who is a friend of mine, but the car is a good example of how club drivers bought and prepared their cars for international rallying back then.
The cars needed to comply with the international homologation papers issued by the manufacturers via the FIA and these listed the mechanical specification for a car to be genuine in terms of competition eligibility. Nowhere did it require the car to have left the factory as the model type under which it entered races or rallies.