Consider yourself lucky....
It would appear you have *possibly* just sheared the gear key on the final drive shaft. This can happen from something as unpredictable as a pre-ignition firing occuring opposite the normal stroke-firing pattern that vectors thrust at just the wrong time. Hopefully the shear pin gave up and it didn't toast the drive shaft.... Anyway, I had a much worse situation where the flywheels, needle bearings and rod siezed on my 70cc at high rpm (well over 40 mph). At decceration, the bottom end of the engine experienced severe oil starvation that immediately toasted the bottom end of the engine. You can't buy just a new rod and bearing set. You have to buy the entire lower end that will cost you about $54 bucks. That's still pretty cheap but it'll take a couple of hours of time to do the rebuild. After considerable testing and modifications I have come to the overall conclussion that these 2 stroke engines are in general pretty crappy. It has taken considerable re-machining and modifications to make this one reliable and get real performance out of it. I'm getting flat out speeds around 50 mph, and pretty decent acceleration, but what I was really looking for was durability. I've switched to a 4 stroke application with a pull start, centrifigal clutch and 2 speed trans. Heavier and less hp but much more torque, smoother, less noisy, no fuel mixing, no clutch issues, simpler to operate and above all else.... durable. The two strokes can be made to work much better, but not by a novice. If only there was a way to get the manufacture to put more effort into overall quality control this could be a winner, but the bottom line is the effort to clean this power plant up, just kills it's practical application. Of course this is just my informed 2cents worth of opinion ;-)