There's not much to rebuild...
The seals will have a number stamped on it.
Roller/ball bearings will have numbers on them...if they are bushings and they are worn you can have new ones made at a machine shop
You can make a gasket the same thickness as the original one (gasket acts as a shim to put the gear in a good mating location with the drive gear).
If the shaft is worn, take it to a machine shop and have a new one made.
If the gear is worn, buy another used PTO unit with a better gear. Just make sure the replacement PTO came off of a SM-420 as the driven gears are custom to the make/model of the trans they are bolted to.
Mike B
