The PTO will be specific to the mower, not the engine as it has to fit into the retainer, plug into the electrics and have the right size pulley to spin the blades at the right speed.
SO you look it up using the IPL for the mower and use the 13 number as your refference.
The clutch will probably come up as an MTD part & not a specific Cub part as MTD make Cub.