yeah, i'd also not recommend a speed kit. they can be great, but they can also cause tons of problems.
if you just want a bit more acceleration and/or top end, then a larger carb (the dellorto 15.15) and a performance exhaust will take you a long way. after that, you can play w/ the gear ratio. but if you go up in gearing to gain top end, you lose acceleration. so just a new carb and a performance exhaust will get you to somewhere around 35-40mph i'm sure.
a speed kit will give you more top end and acceleration, but it might do terrible things to your bike. the other option is surefire. the gains won't be so impressive, but you run little to no risk of destroying an engine.