If you want more power you should:
- Advance the ignition as far as possible. Every little bit the ignition is set too late will cost a bit of power. Setting the ignition too early will cause the engine too run very poorly and the engine will make a "pinking" sound so set the ignition a tiny little bit later than that.
- Increse the compression ratio. You can do this by grinding down the cylinderhead. Do this with polishingpaste on a glass plate.
- A 15mm carb will really get you some extra power. My bike originally had a 12mm Keihin carb, it gave power at about 4500 rpm. I've now got a 20mm Keihin (off a Honda CB125!) and this gives power at 7500 rpm and higher. So that 15mm will give power at about 6000 rpm I think but you really wont lose much power at low revs.
- Set the clutch to engage at higher revs. At higher revs you'll have more power and so you'll accelerate faster. You can do this by mounting stronger springs, they'll only cost a few dollars. If you've got a manual clutch just let go of it slower.
- If you have to pay over $100 for a Leovinci racing exhaust you're being ripped off. In Holland (where I live) those pipes cost about F130.-, that's $60.