if you tune them you will race in open, against bikes kicking out 30bhp - I would just stick to stock class.
140 or 160 are both stock classes.
I race in 140 and the competition is unreal - so close, I race in class B cos I lap Whilton in 58-59 seconds, so far enough of the top lads to not qualify for class A.
Even though, class B is equally tight - it's fantastic fun.