Use time to help you. Let it drain for a few days. Put the drain plug back in and put in the correct amount of kero or diesel. Let it sit for a few days and then drain it. Time will help dissolve gunk. Fill it again and slowly turn the shaft or jack the tractor up and turn the wheels to wash particles off of gears and shafts. Drain again through a filter(even a coffee filter). See how much is caught by the filter. If there is nothing, you probably have it clean. If not, repeat the fill and flush until you are satisfied with the cleanliness. One thing that you may want to do is get a magnetic drain plug. It will hold metal pieces so that they don't move around and cause damage to the gears and bearings. Good Luck, Rick
I like using a thin layer of Form-a Gasket or similar product on both side of the gasket. I have one tractor I did not do this to as the gasket look great. Yep, it leaks around the back so now to do it over again.