The Rotax is an awsome engine, can still win anywhere, the only reason you don't see them is the AMA has the DTX formula (std frame) in the GNC. Spoke to Chris Carr about it, he would prefer the framers as they take up less track (shorter and turn better), therefore in his opion be safer.
Have seen Rotax's win Outlaw races against twins and DTX stuff. The Rotax's is becoming (is) vintage legal and if you front the line with one you know you got a job ahead of you, I referred to them as the natural enemy of the XL350. The ones here have a distinctive bark and will go to 676cc apparently.
Kinda think you are going to hve to keep on top the maintainence though.