Jake, I don't believe that you don't have the knowledge to redo it. You can!
Finding a good used harness is going to be tough. Unless you are really concerned about originality, I usually make my own out of modern wire. I buy a 100 foot roll of 12 or 14 gauge, small package of 10, and a few packages of terminals at an auto parts store for c.$35, and that will do a couple tractors.
You need a wiring diagram and a pair of cutters/crimpers. Assume everything is wrong that's currently on the tractor, and remake it all according to the diagram. If you have the old harness that you know is original and not a hack job, you can make copies off of it.