Hmmm, I am in favour of the 'Pre 90' ruling only so that there is less confusion. Really, as others have said before "Evo is not really an 'aged based' class, I guess only in that the majority (all) of bikes pre 1981 were air-cooled, drum-braked, non-linkage.
Aside from the historic reasoning, it would still be interesting to watch/ride a class is just that 'air-cooled, drum-braked, non-linkage'. Maybe it could take it out of the realm of 'vintage' but I doubt it.
Even if someone put a CR500 engine in a '79 250 chassis, so what, the CR500 is still a 'vintage' machine engine from a 32 year old bike.
I bailed-out of the Evo class because I couldn't use the '84 KX forks on my '79 KX250/400 because they were from a disc-brake model, they were completely stock internally, yet someone with a ton of money could completely revamp the internals of a set of YZ465 forks!
Personally for the sake of vintage appeal, I would like non USD forks added to the basic class ruling :-)