I have used ATF to free motors. Pour the ATF in the cylinders through the spark plugs and reinstall the plugs. Let it set for a day or two and then try and turn over. The ATF will loosen the rust and if starts it will smoke like crazy. If your rings and cylinder walls are rusted as well as any other parts that move in tight quarters, scarring is most likely going to happen. Have a good time with the hone after you get it to turn freely.