How does one work on an old MinnKota trolling motor that seems to be frozen up from sitting for a long time?

I'm not familiar with this exact brand, but in general, when an engine is frozen up, you want to remove the spark plugs and try turning the flywheel manually. Sometimes squirting WD-40 or other oil in the combustion chamber will help free it up. If it still won't turn, you will need to disassemble it. An electric motor is similar. Clean all the contacts, and try to break through the rust and/or corrosion by turning it manually. Check the brushes, rotor, and stator for damage and continuity. You might get lucky and find it just needed a little help or new brushes.