Take starter apart !-----Inspect test it.----Take ohm reading on armature.----If bad you have the smoking gun.----Guess work on outboards often does NOT work and gets expensive.
Put 12 volts directly to the motor.-----That would bypass lots of wires / switches / relays.----If motor works that way then work backwards to find fault.
Very nice boat.----And just like many older products , very easy to rebuild / repair.----New stuff is designed / assembled by robots.----No thought to future repairs.
Not common to go bad.----These were only used from 68 to 72 and most of these motors have disappeared.----Used parts are easy to find , depending on your location.