Same thing I work on every week recently: a ~20 year old Scag STHM-23CV three wheel mower.
Started by sucking ~3 quarts of oil out of the hydraulic reservoir because someone accidentally added oil to that instead of the engine. A piece of plastic tubing fell off of my suction gun and fell into the tank. Thankfully my wife had bought
this kit from Amazon so I stole the grabby drain snake thing to fish the tubing out. Saved me from needing to drain the hydraulics which holds about 2 gallons of 20w-50 IIRC.
Needed to replace the PTO shaft seal. I was worried that getting the PTO clutch off would be a bear but it actually fell off with just a little percussive persuasion. The drive pulley that sits above the PTO wasn't much worse which I probably owe to it being constantly showered in hot oil for the last 2-3 months. Had to drive the key out but it fell right off.
New PTO switch because it wouldn't crank if you shut it down after mowing.
New left foot safety switch because the old one was getting touchy.
New drive belt (third one this year - will probably last longer since it isn't getting contaminated by oil)
New battery
Greased everything
The valve covers had started leaking so I put on a set of the newer style that take the newer gaskets. I should've taken a photo, but the old Kohler Command Pro 23hp is still spotless inside. Runs great too.