I would not use gasket sealer. If you need to, lightly sand the flat metal surfaces to smooth them so they seal properly. The areas where is high pressure do not rely on gaskets to preserve pressure, and the rest is pretty much low pressure of the air system.
If the round ring gasket for the Air piston cover is too small, soak it in warm water for a few minutes. It is made of rawhide and will return to original size.
The quality of the rebuild will be determined by the care in which you place your preparation of the bores. When I rebuild an airpack, I spend more time by a factor of about 5 on the slave bore, compensator bore and the air piston bores than I do everything else combined. If they are smooth and perfect, then everything will assemble nicely and work properly.
Be sure to prelube the air piston cannister walls with Airtool oil.
RL