shaft striped plz help

first of all i have a 84 200s runs great just bought an engine with pull start for 70 dollars. igot it to run and idle great with one pull. i had to put chains on it because it has turf savers on it. my cuz took it out and it was fine then he was doing donouts with the chains on that made me so mad because i told him not to well any ways , when he brought it back it ran for a little while then the shaft for the front sprocket was totaly rounded off. so my ? is that ewould doing donout with the chains on do this , have u ever heard of this and , how would u change the shaft thank you for trailprotrailpro help in advance

[Lots_Of_Nothing]

i think that the chains are your cause, that is lot of stress on the sprocket splines, especially to be doing donuts with chains on, to change the shaft just get a good one, take the engine off, and then take your sidecase off, and then swap the good one with the bad one.......hmm....actually it make be alot harder.....you may have to split your base and replace the whole gear rod....if i'm right.....