One of the design problems with model double acting IC engines like the Snow double acting tandem, is finding a suitable shaft seal. Suitable seals will be polyacrylic, or Viton which are good for close to 300F. They should have a garter spring. They are not readily available from stock in the small sizes needed for models. The seals originally selected for the Snow engine were obseleted and the supply was soon exhausted by Snow builders. The seals that S/S Machine offers are about the only seals available now for a 5/16" dia. piston rod. Smaller ID seals are not offered with a garter spring, and are quite stiff.
The piston rod to piston seal is fine with just a reamed hand push fit. Loctite can be added for a tighter seal, but isn't needed.
You should purchase the Snow Tandem book from Home Shop Machinist Magazine. It might provide some helpful information for your project.
Jeff