I've got an XR75 (5 speed) engine and frame I've always wanted to build into a modern 50 racer. With a few mods and a light rider I think they would kick the shit out of most of the hopped up Cosmo bikes. Mostly due to weight, not power. Should be able to build a 130 lb. XR 75/80 based bike. I think you could get one to hit 80mph on the straight.
In fact....I bet I could swap just about all the parts on Mary's bike right on to an XR frame....we are running a SL frame now. And....actually, you can run an XR in SV 50cc too.