The boom hoist gears are lubed by open gear lube. there should be no covers to take off. If this machine has only done pile driving work, I would not be concerned about the engine rebuild. Chances are it got full of rain water and that is what caused the problem. Does it have a 3 or 4 cylinder. The newer machines were shipped with a 4 and the older ones had the three. Both are good. The 3 cylinder has less power and not so likely to have a lot of broken issues. My 330, is the same as a 305 but has swing out outriggers, larger counterweight and 3rd drum. I have owned two of the these and have extensive repair experience with them.