it could be the reeds, its worth checking out anyway... but my brothers quadracer(which has basically a late 80's rm250 motor in it) did the same thing that you described, it just died while he was going down the trail and wouldnt start unless we pull-started it, all it would do if you kicked it was backfire every so many kicks, we dumped a little 2-stroke oil down the plug hole to try to up the compression and it almost fired enough to start, but wouldnt...
not that this is what is wrong with yours, but his had developed an air leak between the rubber intake boot and the reed cage which leaned it out enough to fry the rings, which dropped the compression enough that it wouldnt start...
so I would say if the reeds are good run a compression test and see what that is, I'll bet it doesnt have enough compression...