Never done one, but thinking about it, I would grind the exhaust lobes off the cam and make the cam run 1 to 1 with the crank then take out the injectors and replace them with one way check valves. That way you have compression on every rotation of the engine instead of every other. A tractor PTO is only 540 rpm so unless you speed it up with a jackshaft you will never get the air you need. But in my world big air compressors are easy to find out of factories.