Get some fresh diesel and dump some injector cleaner in it....Power Services in the grey container will work, or Sea Foam in the pint can. Cleaners are advertised to not hurt the engine when installed in excess and some cans clearly state to up the ante for certain types of problems. Once you have done that hook the tractor to a good load (plow) and work it hard for several hours. Dump a can of Sea Foam in the crankcase and let it idle for half an hour.
Change the oil and filter and when you run out of that fuel, go to the normal amount of PS of SF ratio to fuel.......come back with the results.
No need to go nuts on replacing this and that till you find out you need to.
My 2000 Ford has never had a wrench on the engine at 3800 hrs. I thought it might be time for rings but doing what I said cleaned her up really nice and she starts and runs like a top. Will light off in 5-10 seconds on the coldest days here in N. Texas in December......that says a lot about engine internal condition.