That should be a bucket engine and they are dead easy to work on as the engine stays in the mower
So get a new top oil seal and a closure gasket
Remove both belts from the engine pulley
Then remove the blower housing ( 4 bolts )
Remove the flywheel ( 1 nut )
Remove the alternator ( 4 bolts ) and sit it aside , no need to remove it from the cover
If the crankshaft is nice, clean & shilly then give it a once over with some scotch brite right down the seal , lubricated with lots of WD 40 or similar
Wrap the key way slot with some film wrap to avoid damaging the top oil seal
Remove to top cover bolts ( 10 from memory )
Lift the top cover carefully strait up to avoid damaging the seal or pulling the cams out
Do not rotate anything
remove the dip stick with long pliers
Replace everything is reverse order clean the 4 bolts at the front of the engine and apply blue locktite to them .
I do a gasket in about 2 hours so allow you & your other half a full day
Because I am an old style mechanic , I put blue hylomar on the closure plate then use that to sick the gasket to it and a thin film of axel grease ( some times called bentonite grease ) on the crank case so in future the gasket will come off in 1 piece with the closure plate so can be reused as they are a little $$$ .
The really hard part is resisting the temptation to rotate the crank or cams while the plate is off thus potentially getting the engine out of time