Alot of the garden tractors that were made for other garden tractor manufacturers had a slightly different setup when it came to Deck/attachment engaging.
It might of had some sort of contraption on the deck that disengaged the belt.So the PTO would spin all of the time.
Do you have the original deck or any other original attachment for it?
If you look in the one picture you can tell the PTO Idler was there at some point. The paint on the tube is worn off right where the Idler should be. Thats a sign the setup was intact not too long ago.