If you recollect the on going status of the Colorado class once, not too long ago called Formula Colorado you will see that there's very little
interest in it. With even the Lightweight GP class shrinking as everyone's getting out and getting a 600 or above bike. Funny thing though as I know of at least two riders who have downsized to a lowly 650 twin and are having great fun again.
If you interested in racing a RD-350/400 or anything vintage give www.ahrma.org a gander. There's no room here.
I do know of another rider and myself who are interested in racing converted dirtbikes take a gander @ www.450moto.com Looks like great fun to me!