Luckily, your tractor is old enough that,, I believe,, it was built before the computer was invented!!

There is not much to the harness,,
I would probably simply hand rewire the tractor,.
Any available harness is as old as the tractor, and,, if not already,
will soon have the same brittle wire problem you already have.
Starter, glow plugs,, headlights,, that is about all there is.