On a truck that old there is no "piston" on the PTO you describe. It is mearly the shift fork and slides in/out by the lever it is attached to. There is nothing else inside the PTO except the gear which slides on it's own shaft to engage to or disengage from the drive in the tranny. Try one of the Chelsea PowerDrive dealers in Portland.
Be sure to replace both seals while you have it apart. The PTO gaskets come in many thicknesses so measure (mike)what is on it an replace to same thickness to ensure the proper backlash. Check the studs while it's off too, cheap insurance.
