I agree with "Slick" on the Marvel Mystery
oil. I've used it on numerous motorcycles I've purchased that, for all intents and purposes, were locked up tight from not having been run for years and stored outside, behind the shed.
You squirt it in each cylinder and let it sit for a week. Careful pressure on the crank with a breaker (that's if it is totally frozen) and it will eventually yield. I've recovered full ring seal after running the motorcycle for a couple hours after the MMO treatment.
This was recommended by a professional drag racing motocycle engine builder and it has worked for me on 3 frozen motors. Don't know why it works, but it does. WD40 may indeed work but it's a solvent as well as a lubricant and if it isn't recommended for guns, why would it work on motors? MMO can be used as a top cylinder lubricant through the carb so it's a better choice, in my opinion.
Paul