The free fix is one of these. The gas/oil mix, air/fuel screw and the throttle idle screws just need to be adjusted to increase the peak RPM. Also, your choke might be getting bumped with your leg.
Is this at top speed this is happening or is it at any speed? Please clarify. I'm just tossing lots of ideas out there... Assuming you mean top speed/peak rpm...
Not enough fuel is making it into the chamber - get a bigger jet for your carb. Also, getting a better exhaust will make a world of difference in performance. As well, you can go with a smaller sprocket for your driven chain, this will force your bike to torque more before hitting peak RPM. Get some better oil, look for a synthetic 2stroke blend, and stop looking for cheap walmart deals.
I have a 66ccRAW at my feet now, just need a bike to toss it onto... it's got a 76 Delorto jet, I really wanna fire it up and see if it still runs, it's been sitting for a year and a half.