Nasty Surprise

[CaptainCazador]

Shouldn't be a major issue, can't think why you'd want to take the engine out, just take an angle grinder to the weld, change the sprocket and weld new nut on.
It was the solution I resorted to before Yamaha recognised there was a problem and came up with a deeper nut. I changed the C&S twice more after the first welded-on nut.

Or worn splines on the output shaft.