Rings are Mercury pare # 39-21665
New piston scarce and $$$ A good used piston will work
Upper crankcase seal 7415 (industry code)
Whether you do a little or a lot you will need gaskets and gasket sealer
There are several sources for used parts
Fergusson Poole has gaskets and probably rings
Water pump has a seal and impeller which are likely to be iffy at best.
I believe there is a printable copy of WD4 parts manual which will give
you a good idea of WF4 and WG4
Keep us posted on progress and problems.