Go to a electric motor repair/rewind shop tell them what you want they may give/sell you a brush large enough to get a piston out of or in my case 10 pistons.
Gary
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)
I have tried all of the suggestions that I have found here. None have helped. I use two different android boxes using up to date versions, neither will stay logged in.
Gary