Out of curiosity - dopes your tractor have a shuttle shift? If so does it have a PTO also?
I used to work in industrial maintaince in a plant in Massachusetts. We used to go to a rubber place in Zyer that had any rubber seal you could imagine,they cut it special order in many cases from bulk stock. Maybe you can find one in your area.If nopt e-mail me and we can see if I can find them so you can call them.
Good luck Mr Bill