There's guys using Stihl chainsaw cylinders, minarelli vertical cylinders, custom clutches, billet cases... All kinds of shit.
I race with them. They're making bikes that'll do 50 on the track, and 70mph on the street. This guy Robert Bianchi made a dual, side by side cylinder motor. They keep making gnarlier shit, long after I gave up and went to the V1 (which still beats them on the track.)
You can check the "socal motorized bicycle racing" group on Facebook.