It's not a real bad job to change the shifter shaft, if you are mechanically inclined. But it's not that easy, either. Both, the right and left side covers will have to come off. The left side is easier, because there's nothing much to fall out of place. But there IS a tiny pin that engages the shift drum that can fall out, so you better watch close, in case that happens.
The right side cover, though, is a different story. The kick starter shaft will try to fall out and the reverse lock out shaft will try to fall out. Also, all the little parts that the internal clutch lever engages will fall out of place. The kick starter is probably the hardest to put back, because if you don't get it back right, it will damage the gears on it.
I don't consider it that hard, but I have done it a lot of times. It may take you a few tries to get all the parts back right and get the right side cover back on. The reason that the right side cover has to come off is because there is an internal clutch lever that has to fit over the end of the shift shaft. The shift shaft has splines that the internal clutch lever fits on and it has to be timed. There is a punch mark on the internal clutch lever that has to line up with an index mark on the inside of the crankcase, under the side cover. Once you pull the shift shaft out the left side, the internal clutch lever will fall off.
It would probably be a good ideal to get a repair manual if you attempt this yourself.
