Sounds like bearings are worn out. Tractors will run a long time in the condition you describe if it isn't knocking or making other unfamilar noises.
I once had a car that had zero oil pressure after warming up. I added one can of STP to the oil and the pressure would only drop to 20 when warm. At low RPM, it was still pretty low. STP today is not as viscous or sticky as it was when I did that so it doesn't work as well. Back then, the engine had to be at operating temperature to add it. It is still worth a try.
Beyond that, you'll need to replace cam bearing and all crank bearings. Usually the crank can be reground and under size bearings can be purchased.
I'm not a proponent of most of these oil additives and believe some do more harm than good. But STP has always served me well.