You probably know that your motor has to be in forward gear to get the throttle to move past start. This is a safety thing so you can’t start it at max throttle in gear.
Also – as Oldlure mentioned, that is a 6 volt motor unless someone has changed it to 12 volts. This is done by replacing the choke actuator and the starter solenoid. The 6 volt starter motor will work fine on 12 volts. The starter solenoid is in the metal box that mounts in the boat. Both parts will likely work for a while on 12 volts, but will go bad sooner or later. The 12 volt starter solenoid you need is at the link below. The choke actuator can come from a 1957-58 35 HP, or a suitable resistor can be put in series with it.
Dave
https://www.marineengine.com/newparts/part_search.php?part_num=18-5807