If you have a dry place to store the engine, I would remove the cylinder head and add some penetrant to the bore.
Most likely, the iron rings are rusted to the iron cylinder. ....Removing the head may allow you to see the condition of the bore.
If the piston is part-way down the cylinder, fill the cylinder with penetrant or oil, and let it sit for a week or two. ....You can periodically add more penetrant as needed. ......After a week or two, try turning the engine by hand. ....Do not use tools to force rotation, since you will be doing this project at a later date. ....Instead, add more penetrant and give it more time.
You can try motor oil, WD40, ATF (automatic trans fluid), rubbing alcohol, or a 50%-50% mix of ATF & acetone, or commercial penetrants.
The engine could also be seized at the crankshaft/rod journal, but more than likely the problem is in the cylinder.
Another place to check is the magneto armature to flywheel gap. ....I have seen rust at this location prevent rotation.