Seems like miniGP is finally catching on in the US. Kinda different here than in most places though, here it's mostly kids dirt bikes outfitted with street tires. High end pit bike racing is huge in Europe and churning out bad ass young racers. Now there's a few companies putting those engines in tiny GP bikes for adult training and to get kids into road racing.
I got to sit on a couple of these Ohvales at EICMA in Italy this year and they're pretty fuckin sweet. They make like 18hp at the wheel and weigh 150lbs with a full tank. They're like 6 Gs though which seems kinda nutty- you can get a nicely set up race ready KTM rc390 for the same cost!