It's fairly easy to do, but first you have to get at it.
Split the tractor, remove the clutch and then the flywheel and there you are!
Since you have the pan off, and before you go further... I'd add pressureized oil to the gallery to see where most of it goes. It may not be the a pump problem at all. A quart of oil in a garden sprayer can tell you a lot.
Another possibility is a slight air leak in the pickup tube, allowing the pump to lose its' prime.