In addition to ALL of the other items mentioned in the replies, I would recommend that after you change oil that you pump the same type and grade of engine oil that you are going to use in the engine into the engine at the "T" on the right side of the engine block until it comes out of the valve rocker arm pivot shaft and rocker arms and until you get at least 15 PSI oil pressure. (I use 10W- 40 Synthetic oil in all of my equipment.) Be sure to FILL the crankcase to the FULL mark on the dip stick. Most tractors take more oil than auto engines. Install a new oil pressure gage (of the proper pressure range) if there is any question about the old oil pressure gage.
I also recommend that you run the first tank full of gas through it with a "chain saw" gas mix of 50 to 1 with synthetic 2 cycle chain saw oil in the mix.
When checking the valves to be sure that they are not stuck, oil them and the rocker arms & pivot shaft very well.
Be sure that the clutch is operational (not stuck) for safety.
When you install a new oil filter be sure to install the correct kind and type or you will not have proper oil pressure.
Hope this helps, Gordon