very little difference between the g's and j's mostly cosmetic bodywork.
im racing a g2, it had a modified head, very little to do with them except 2 mildly polish it as they are very good as standard, but i dropped a valve so only have a standard head in now.
but have kent short track race cams in, usual k&n, full yoshimura system, didnt go down the dynojet route as they only do a stage 1, so got a factory kit from the states along with an adjustable timing rotor which runs 4 degree retard.
bikes making 115bhp at the rear.
spend some money on the suspension, i was told the probs with these bikes and why nobody races them is they are hard to set up, rubbish, ive had the front forks rebuilt and sprung and the same at the back by kais and the bike handles fantastic, really positive into and through corners, will carry more corner speed than i can use!
hope this helps