In some dirt track meetings that I have attended EVO is just air cooled, no links, drums. This leaves it open for ALL twin shock and triangle mono's generally up to pre 85 which allows a lot of blokes to ride up with their older bikes.
If you make it an era then you will loose a lot of riders in this class.
Whats so wrong with mixing it with the older rigs, in mx their is not too many older bikes that can hang with what some would describe a a "genuine" evo bike anyway.
In saying that I do know of a few specials with aftermarket frames, billet triples etc, etc running in pre 75, if thats what you need to win then go for it, but it just makes it more rewarding when you beat them on a stocker.