Try soaking it alcohol fuel. You may also try heating it up with a heat gun. The engine is probably gummed up from old Castor oil residue. The Castor will soften with heat. The alcohol will also dissolve the Castor but takes a long time. I would take the back cover off when soaking to make sure everything is well submerged.

The bearings may be bad but may also be good. Get it loosen up first. Seals may also be bad especially orings like found on needle valves and around carb intakes.

Take your time.

Cheers,

Chip