Use a brown paper bag for gasket material.
I have used RTV silicone gasket sealer for thin gaskets. There are several that will withstand the high temperatures of steam. If the engine is to be run only on air, standard RTV materials will work just as well.
For my Stuart 10 H & V I drew the gaskets in CAD and printed them on drafting vellum. Cut them with paper cutter and scissors: nail scissors from my wife worked great on inside cuts. Holes were cut with thinwall tubing turned to a sharp edge and run in the drill press cutting into the paper on end grain pine wood. Vellum is thin and strong.