If there are enough splines left to drive the sprocket, find a spacer, or get one made, to go on after the sprocket to bridge the bad part of the thread, Only make it as thick as it needs to be.
Fit a new oil seal, use a new sprocket, your new spacer and a new sprocket nut with red Loctite. (You will have to heat the nut if you ever need to remove red Loctite!)
This may or may not work. You will have to split the cases to change the sprocket shaft.
Show us what you're working on (MT or very very closelyy related subjects only please).
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