PB Blaster or any other penetrating oil might help.
The motor is held on with two bolts/screws. You have to first remove the arms and they are held on with a retainer that has two clips and a spring between them. I think you push each clip towards the outside to remove the retainer.
Note:
"Use caution when using PB on: plastics, paint, rubber*, fiberglass, belting and similar materials. Always apply to an inconspicuous area before proceeding with application as PB may stain such surfaces.
•The petroleum solvents in PB will impregnate common black butyl rubber. If left in contact with these materials for extended periods of time they will begin to swell and deteriorate. However, these solvents evaporate and are not typically on applied surfaces for long periods. Components such as O-rings, control bushings, and gaskets, made from these materials get dry and hard after years of use. The penetrating/impregnating qualities of PB Blaster can restore some of the elasticity and softness to these old hardened components. "
