Has anyone ever replaced the main self tailing winch on an Oday 34 or 35?
Welcome to the SBO Forum Bob.
Replacing a winch is a common task for all boats, including the Oday brand. It is like replacing tires on a car.
A picture would likely help some of us older sailors to identify the brand you have on your boat.
Like tires, there are a group of manufactures that product the hardware then sell it to boat owners or boat manufacturers.
Depending on the age of your boat he manufacturer may be out of business.
@Dave Groshong has provided info about two of the better known manufacturers Lewmar and Harken. You can source your needs with him. Using the SBO Shop helps to maintain this fabulous website.
Alternatively, older winches made by Barient or Barlow (who were prominent in the 60's through the 80's) made winches that are still in use today. Certain internal parts can be found from sources like Lewmar.