Delrin (polyacetal) would be my choice for a small air engine. Besides being good for highish temps (>100'C high for plastic) it is springy and allowing for that can be made into its own seal that doesn't need lube. Its very slippery on polished metal.
Something like a cupped piston would work fine as the cupped part could be thin walled and the springyness of the material (maybe compined with the air pressure) would make it a good seal, without the drag of orings or other seals. Its a plastic commonly used in small pump and valve type devices because of those properties.