Jumper Direct to the big starter lead and it turns - good starter motor
Jumper dorectly to the solenoid connection - Click heard, starter spins - Good starter.
Neutral lockout - if installed
Panel power - if a panel is installed
Panel push button switch - if installed
panel key switch - if installed
Starter neutral interlocks are usually fail open. That is if the wire fails the engine won't start.
diagram for you boat would be useful but being so old it probably doesn't have one.
You might want to diagram it for yourself or trace the wires to see what equipment