Stuck motors can be stuck pistons, stuck bearings, stuck gear case and
can range from easy thru impossible. Every stuck motor is different. It is
best to have a parts motor to source replacements. If you are lucky between
the two you will have enough to make one that runs and looks good without
excessive investment.
Proceed with care, take notes and pictures as you delve into problem. Treat it
as an exercise in patience between you and motor...first to lose temper
loses all.
Louis