If it is an SP Mk 5 clone or a Mk 10 clone, the odds are you will be able to flip the HP seat over as pretty much everyone used flat seats at that time. The two holes on the end of the first stage are for removing the seat retainer. An SP multi-tool would be ideal but just about any adjustable pin spanner will work. Once the seat retainer is removed, the best approach is to blow the seat out of the retainer using compressed air and a small rubber nozzle sealed against the hole. That will prevent damaging the seat.
If the seat has already been flipped, you could possibly reface it with an emory board as long as you keep it square. But be aware that the thinner the seat becomes, the higher the IP gets. A better solution would be to find a new seat and that is probably not as hard as it sounds as it probably uses the same seat as the SP MK 5 or Mk 10, the TUSA MK 10 clone and various other Mk 5 or Mk 10 clones.
You will also need a specilaty tool to remove the cap in front of the yoke to access the and remove the piston. The multi-tool will again work fine to accomplish that.
The yoke nut requires a specialized yoke nut socket that is both shorter and thinner than a regular socket. In a pinch, if it is not corroded in place too badly, large (15")cresent wrench can be used. The downside is that you will only contact two of the flats on the nut and can damage it if it is stuck in place. And if you get it removed, you can't torque it properly with a cresent wrench when you re-assemble it.