Thanks for the replies.
We have to stay 50cc to meet the rules unfortunately.
We've already removed 3 of the weights, would drilling them to be lighter make a difference, or is it just a better engineered version of the same?
We have tried a pipe, the bike came with what seems to be a Turbo Kit S1 pipe from Moped division. It seemed to make it run badly, and caught on the stand so we went to a standard box.
Nick, did you find an improvement with your custom box? We'd be up for making one if it helps, is it just empty inside?