Mooch, you do realize that this is exactly why AHRMA, the AMA, and anyone else that has a vintage race or series has such a hard time. No matter what you do, there are people / bikes that don't fit in well and they raise all kinds of greif for everyone and want another class where "their" bike is not the "turd" of the class, but a viable weapon against other bikes in the class.
It's a one off race for some fun........ bring what you got and run it in something. It doesn't have to be capable of winning the class to have fun running on a grass track in front of a hotel, just have some fun. If you are in need of a class that fits your bike perfectly, go to an AHRMA event instead, they make sure that your precious YZ fits in the right age group.
I doubt that anyone will "protest" you and take away your prize money for riding a 1981 YZ in the vintage class, except maybe me cuz I have a 1983 dual shocked and aircooled Husky 510 TX fourstroke that rules in the Air Cooled, Dual Shock Class!!!!! Don't make me use the "his bike isn't dual shock bike card at signups!"
Just kidding, have fun and ride for fun.