Check the alternator first, you don't want to go further if you don't need to.
Those 6 rivet units can be a problem and there are numerous bikes that have had them replaced with the 12 rivet version. While I don't own a bike with that shaft I have been into one of my k75 engines to work on the output shaft.
If you have a helper and a few tools beyond the bike's tool kit you should be able to service your output shaft without too much trouble. I would guess that if you work straight through it's a very easy three day job. While you are in there it's a good time to do other things that should be done like clutch o-ring, rear engine seal, service the sprag clutch, and the anti-backlash spring. When you're done you will have addressed all the potential issues you could have with your engine.
You don't have to go into the engine itself which makes the job a lot easier. Basically, you drop the oil pan and the bottom half of the crankcase. Once the engine is out of the frame, I would guess the job would take about 6 hours or less. I've read about an owner who did the job with the engine still in the frame. I suppose it's possible, but there isn't a lot of room under the bike to work. There was another owner who said he did his by laying the bike on it's side. That's not a bad idea in that I would guess it would save about 4-5 hours removing and reinstalling the engine in the frame.
