For the fuel supply checks:
Start when you are replacing the fuel filter since you will have to bleed the fuel system of air anyway.
Since I don’t know what type of fuel filter your tractor uses I am assuming some type of spin on fuel filter but a similar process is applied to all.
Fill tank up to at least half full or more.
Get something clean to catch the diesel fuel in.
Take the fuel line off before the filter and let it drain into the container and see how it flows for a few seconds to a minute. Is there a full hose size flow coming out or does it slow down or seem slow out of the line? If flow seems good then move towards the injection pump to another connection of the fuel line and repeat.
If first test point is slow move towards the fuel tank and repeat test.
When you find good full flow out of the lines you kind of know where to start cleaning or replacing fuel lines and filter housings or filter.
If there is poor flow directly out of the fuel tank outlet then you can puff a low pressure (5 psi) air for a small amount of time (.5 to 1 second) to see what happens to the flow out of the tank.
Eventually replace the filter and bleed system of air however that is done for your tractor.
Report findings and more assistance will follow.
Wear “latex” gloves and try to be neat unless you like the smell of diesel on your hands for a couple of days