[Sid Squid]

I suppose it's just possible that if you tidy up the thread then the nut might just hang on to the remaining metal, but I don't hold much hope of that. Sadly the only way to fix it properly is to replace the output shaft.

Some shifty sods have been known to weld the sprocket on, this will work in the short term obviously - but it's only putting off the inevitable shaft replacement.

It may be possible to repair the shaft, perhaps metal spraying and re-machining, but I rather doubt this will be cheaper than replacement, and I can't see anyone doing it in place anyway, so the motor will need to come out and apart, in which case a replacement shaft, or even complete S/H gearbox will be simpler.