I've run into this same problem with some of my high-mileage bikes. Keeping the shaft well greased is about all you can do. I looked at options but I agree with 2door, there isn't enough meat in the casting to bush it effectively in my opinion.
But then, I've worn out clutch levers from riding so much!
If more is better, then too much is just enough.
If you can't afford it, build it yourself.