I’ve always had great success using acetone to free up a stuck engine run on castor laden fuel. Much better than using methanol or mixed glow fuel. I usually heat the engine up a little first to get the carb and crank moving a little bit, then add solvent and keep moving things. After 5-10 minutes, dump it out, rinse, and add oil. I use ATF mixed 10:1 with Seafoam - works a great for keeping carbon deposits at bay when used as after-run/storage oil.

Also; the very best penetrating oil I’ve used is a mixture of Acetone and ATF mixed 50/50. I keep it stored in a metal can with a tightly fitted cap to keep the acetone in - much like model diesel fuel.

just my 2¢.