I have a 1989 Horse that I've been taking care of these 20 odd years. Back in 92 I had a bearing in the main transmission break and back then, I got parts from Troy Bilt. Nowadays, Troy Bilt is owned by MTD in Ohio after the former went bankrupt at the hands of a liability lawsuit due to their "Tomahawk"
chipper shredders. MTD still makes the Horse, though the other PTO attachments such as the log splitter, generator, and
chipper shredder have all been discontinued, so happily, Horse parts are still available via MTD.
It sounds like your worm gear and/or worm wheel in the tiller unit is gone. Mine hasn't worn out yet, but I think it's time is coming.
There are many models of Horse tillers on file with the MTD parts lookup engine, but the PTO horse model's mechanicals haven't changed much over the years.
Try this link:
HELP FINDING YOUR PART
then follow the "illustrated parts lookup tool", searching for "Horse." It helps to have the serial number which is stamped on the top, right of the main unit's transmission housing.
Most likely, you'll need the worm wheel, and maybe the worm gear with it's associated shaft, along with all the gaskets and oil seals shown on the diagram. I kind of hope you won't need the shaft with the worm gear, however, as I bet that's a pretty expensive part.
Good luck!