The output shaft bearing is a large bearing and it would take a lot to wear it out, I've seen bikes that have covered hundreds of thousands of miles and the bearings are solid.
A rare knackered one would have and up/down or side to side movement.
You say half the splines are worn, was an ill-fitting sprocket fitted at some point ..?
You have two solutions , fit a new spline or weld a new sprocket to the shaft.