a suggestion to the OP: how high do you rev? the peak power is around 6500 RPM for a 66cc (if I remember correctly), and just as with a car, shifting to keep within that powerband is key to speed, HP tends to drop off pretty steeply after peak so shifting just after that RPM you may see more speed. My bike does 40mph with a 36T, and I plan on getting a shift kit for launches and more speed (I'm tuning for high RPM power, because I have gears, a single speed you want medium to low end power, but as broad as you can). a dremel and some JB weld can help, but since you have a shift kit, reeds for the entire RPM band, and a short(ish) expansion pipe (I imagine the banana one would do just fine) as well as tall porting you should see your powerband lean more to the high end, launches will suffer though, but from what I understand, your average bike casette's lowest gear is roughly equal to a 56T sprocket, giving you around 50% more torque minus losses anyway). Also a jag CDI (or build one yourself) to help with the spark. looks like you've already shaved/lapped the head so you're pretty much already at the limit for what you can do for compression for free.
