The AMA's Formula Xtreme rules essentially allow the creation of 600cc Superbikes. The engine case, frame, swingarm and other associated items must remain stock, but modifications are pretty much "run whatcha brung." The American Honda racing team did such a good job at building the FX bikes in-house (and winning the 2004 championship) that HGA gave them the green light to do the same with the Superbikes in 2005.