I'm not suggesting you dont trust them anymore... I would however make sure they understand the possition this has put you in(you cant use your tractor) and how imperitive it is that they make it right(you've already spent time and money trying to fix their guaranteed product)... Bottom line is that things happen, this may be an oddball deal for them, but you are still the customer with the warranty.. They sold you a product and gauranteed it to be reliable for a period of time..
The whole distance thing makes it tough, I ran into the same deal with mine. When I took the 2000 back for the 50hr svc, I had a list of things I wanted addressed including several leaks.. Most of it was handled when I got it back a couple weeks later, but I still had a leak from the L/R axle seal.. He told me to bring it back, that was good enough for me, and I told him I would fix it myself(I'm not scared of working on it either) since it was a 1hr drive each way, equaling 4hrs total and gas money as well.. I let him know that I was dissapointed in the repair, but basicly released him from any further warranty repairs at that time.. When you buy a used anything, eslecially if it is 30yrs old, you kinda expect to work on it at some point..
Good luck with it, I hope they do you right...
If you have flow through the radiator, clean radiator(outside/fins for airflow), fan turning, water/coolant and an accurate temperature sensing device, its pretty much covered..Have you touched the radiator(be careful)? run your hand, closely at first to avoid burns, but see if you have hot spots or worse yet, cold spots.... Have you checked the oil? Is the level correct? is it contaminated? How about your air filter?(kind of reaching here).. Are you blowing any smoke?