Get a gallon of a good solvent. like paint thinner, turpentine, penetrating oil like WD40, Kroil, etc. Get an empty gallon can, cut the top out of it, put the whole gun in the can, and fill the can with the solvent. Let it soak outdoors for a week or two. Take it out of the can, let it drain for a bit, and then try to remove the sideplate screws and the the mainspring screw, which will release the mainspring. Use a piece of wood as a space-bar, place it in back of the thumbpiece, and tap it with a hammer. If it moves, work the thumbpiece back and forth, loosening it up. Once loosened, tap the side of the cylinder with a rubber mallet, and see if you can get the cylinder open.
Keep proceeding this way, until you can get the entire gun dis-assembled. As parts come free, rub them with a oil-soaked 4-0 steel wool, removing any loose rust and any sticky matter on the surface.
Mike Priwer