It is 50 ft lbs.
I lost mine once. Did everything right, torqued it, bent the tabs over... The worst part was I had the stock cover on, the sprocket slid out onto the threads on the shaft and tore them off to where I had no chance of getting the nut back on.
I drilled the end of the shaft and tapped it, put a spacer in to hold the sprocket in, Lok-Tited a 3/8" grade 8 bolt in the end of the shaft. Rode it this way for over a year without loosing another sprocket. I did have trouble with loosing some oil at the seal, I assumed that I didn't get my bolt tapped in dead center and this would wear out the seal.