The class is divided by ability - novice, intermediate, and expert. But not by engine size. Historically the club has introduced further divisions as and when necessary. However you'd have to be a bit of a nut to race a late 80s CR500R anywhere other than the desert...right?
In my humble opinion (IMHO) you should buy a "historic" bike because you do have an emotional attachment. Unless you are lucky to pick up something really old but cheap and in good condition, the economics are all upside down so that has to be offset by some other "soft" value. I am confident my CR250R project has consumed more time and money than buying a 1 or 2 year-old modern bike. But then there's the cool factor of practicing at Elsinore and no one's old enough to even remember twin shocks, the camaraderie at the club, on these boards etc. I think the sweet spot demographic here is the 40+ guy who wants to re-live his youth, but not kill himself physically or financially. For the Decade class that means we should see an up-tick in interest in the next couple of years...
I also think there is an un-tapped market for guys with responsibilities who want to race but not end up in the hospital from tackling today's new school SX tracks, but that's for another thread!