That is a relief: it means there is definitely leakage past the rings, and the piston has to come out - no risk of unnecessary dismantling. That requires removing the engine from the mower. The most difficult part of that is freeing the drive shaft, by sliding the clutch along the shaft toward the chain-case on the left side of the mower. The right (engine) side of the clutch is usually held by a captive clamping bolt, which just needs to be loosened, and can't be removed anyway because it's captive. The retention of the left (chain-case) side of the clutch is obvious and less tricky, though at this moment I don't recall what kind of screw/bolt it is. You'll need to disconnect the speed control cable at the engine end. The engine is held down by four bolts, which are easily visible.
Sometimes the clutch is rusted onto the shaft and it's a bit tricky to get it to slide along. Soaking the shaft/clutch hub where one slides into the other with solvent is the first step in that process. If the shaft is rusty it will need to stand overnight to let the solvent take effect. Various solvents will work - Penetrene or WD40 for example.
