thread size on output shaft

[rockgator6]

I recently bought a 97 warrior and the front sprocket is missing the lock washer and the nut. the sprocket has been tack welded to the output shaft. there are still 2 or three good threads on the shaft, i am going to grind the tack welds off and see if a shop can cut new threads on it. also it is leaking oil between the head and the engine case, can the head be removed with the engine still in bike or does engine have to be removed.
I have ordered a clymers manual for it so that i can fix the oil leak.