Other people can start manufacturing the parts if the demand is big enough and the designs are available or can be reverse engineered.

Not familiar with this world, but they could use relatively standard, widely used parts to build the tractor. For example, Yanmar engines are widely used in the marine world. The engine choice is abstracted away from the boat (for certain types of boats) and you don’t rely on the boat manufacturer sticking around. Everything can be retrofitted.