Bob, I had the same issue on my tiller gear box.
I had a machinist friend of mine make me a new down shaft. Jeff in pa makes them as well as
http://www.xtrememotorworks.com.
One thing you could try is to replace the key stock. I would have done that if my down shaft would have been in better shape outside the gear box. Didn't do anything to the side shaft and it tightened mine up great.