I had a similar thing happen to my 01 yz125. I don't know how similar they are (125 - 250) but I think what happened was that when my son gave it a kick to start it, he didn't use enough effort and the bike started backwards on the rebound. It ran for a very short period of time, made some noise and shut off. It did not split the kick shaft but it did break the crankcase cover and split the splined tab (splined to the inside end of the shaft) that fits in a recess of the case cover. That is what broke the case. I ordered my parts from www.cycle-parts.com, 86 for the case cover, 36 for the shaft.
Hope this helps,
Mark