a flywheel weight will do the opposite of what you want.
i'm not sure what year your 80 is but dennis kirk has aftermarket pipes from procircuit and fmf. but if you want to keep the stock pipe:
1. i've never tried this but if you put o-rings between your stock pipe and the engine it'll give you more bottom end power from stock. the more rings-the more bottom end, but it will sacrifice power on top.
2. i hear you can do this with a cr125 silencer, but maybe you can cut down the silencer on your 80, that'll also give a bigger hit down low.