I have my great grandfathers WG4 1954 wizard outboard he handed it down to my grandfather and now it’s mine it’s been sitting in my grandfathers shed for the past 10 years I brought it home rebuilt the carb got it running ran water through it the best I could then put it in 55 gallon drum with water and cranked er up to let her run and get the carb dialed in unfortunately the water jackets were clogged and she over heated when I pulled cylinders off I noticed a ring was broke and the piston was scarred luckily cylinder wall was unharmed trying to find some sort of rebuild kit or new power head or pistons and rings
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.