Since the tractor has never worked in your hands, I wouldn't assume that anything is good until proven so.
I recognize that you have been into the fuel pump, and I give you the benefit of the doubt that it is ok, but that is an assumption that I think ought to be questioned at some point.
I would check the injectors; either take them to a diesel shop or build your own tester (there are reasonable instructions out there).
If they pass, I think that you need to check the fuel lines for air leaks as that will definitely cause an overfuel/overspeed situation. These engines seem very sensitive to small air leaks. And I mean tiny air leaks! Look for dirt on threads, a hair on the fuel filter gasket, diesel weeps, cracks on fuel lines and fittings. Use lots of compressed air and brake cleaner, with brushes and rags to get things spotless. Given where things are, I would check for tightness every fitting on the fuel system from the tank onward. If that doesn't fix it, I would disassemble the system, replacing any rubber that doesn't act new, the fuel filter... I would assume that the prior owner left dirt somewhere until proven otherwise.
It may be easier for you to do the fuel system overhaul first, but it is more work.
There are some ideas on this older thread;
Post in thread 'Governor adjustment'
Governor adjustment
All the best,
Peter