Utahsba has a similar class, Twins GTO. The SV guys still have their own class, Twins GTU. The only addition to the this GTO class structure would be to allow the GBR 675's in. Wmrra runs a Formula Thunder class, similar idea.

Utahsba Twins GTO class structure
a) Up to 430cc two or more cylinder two stroke, unlimited origin
b) Up to 550cc two or more cylinder two stroke, if frame is from a motorcycle manufactured for street use in North America
c) Up to 550cc single cylinder two stroke, unlimited origin
d) Up to 1000cc two cylinder four stroke four or more valves per cylinder, liquid cooled, except the Ducati 999R and equivalents (Ducati 999 is legal in this class; Ducati 999R is not)
e) Unlimited displacement air cooled, one or two cylinders


WERA has a class structures similar to the "Misfit GP" idea.

HEAVYWEIGHT TWINS SUPERBIKE EXPERT & NOVICE and HEAVYWEIGHT TWINS SUPERSTOCK EXPERT & NOVICE

Unlimited displacement air-cooled, four stroke twins, (based on Formula rules in HWTSB)
Unlimited displacement water-cooled, four stroke twins, (under 900cc based on Formula rules in HWTSB
Up to 700cc water cooled, four stroke triples
Any machine legal for Lightweight Twins
125cc & 250cc GP Machines in Superbike Only
*Up to 1050cc Triples based on Superstock rules will be allowed in HWT Superbike only.
*There are no bodywork limits in HWT.