gildomilo,
I don't have any direct experience with these units, but while researching sources of owner + parts manuals for my Chipper/shredder, I found a site that has scanned copies of discontinued Troy-Bilt equipment. There are detailed Master Parts Catalogs for Horse Models that may be of interest to you - if you don't already have one. You'll find a 180 page manual for 1977-78 models followed by a 44 page manual from 1988. Link's listed below -

Here is a link that might be useful: Troy-Bilt Manuals

Call this number and Bill can help with parts and your problem Bill 1-800-609-5110.
The worn bearing is the rear one and should be replaced. Remove the tine cover
and the rear cap. Drive the main shaft to the rear using a "Dead Blow" or shot
filled mallet. Don't use a metal hammer or you will damage the shaft. The
bearing will come out with the shaft. The front race should just slip out. Drive
the old bearing off the shaft and press a new one on. You can also slip a new
bearing on the front if you choose.

Remove the seal from the front cover and replace the cover and bearing race.
Slide the shaft in until the front bearing seats in the race. Slide the rear
race in and add shims as required. Bolt the cap up. Test for end play and add or
remove shims as necessary. Once everything is ok, install the new seal at the
front, add gear oil and go. I can supply the parts. Bill 1-800-609-5110

How much would this cost the way your describing..