welding on main shaft?

You won't get the required finish by turning down on a lathe; you need a grinding attachment to a lathe to achieve the right finish.

Your best bet is to get another one if it is a bother.

Next would be to remove the rust in the pits and finely polish the edges to get rid of any burrs and try that.

Next would be to turn down the shaft snout, turn a 'ring' of proper hardness material for a strong interference fit on the turned down snout, press that on, and then final finish grind the new ring to new dimensions. This was an unoffical but effective service procedure used on early Saab 99 mainshaft snouts that tended to wear and fail prematurely.