I built one years ago the power piston is graphite in an acrylic cylinder seems to work.. the displacer shaft is just a hole in long (1 inch) acrylic, brass shaft. Runs best on a bowl of ice. (Built from a photo)
On aircraft engines we take the plugs out hot every year or 50 hours very rarely is there a problem. When they go back in we use a grafite based antisize sold by Champion and torqued with a calibrated tool. I think the special spark plug antisize is the key.