Depending on the year of your boat you probably have Eaton Heinemann type AM1 magnetic-trip circuit breakers that act as starter circuit switches (15 or 20Amps). AM1 single pole breaker's physical size is 3/4in wide x 2 7/16 long with two studs on rear for electrical connection. The stud size is also standard and I think it has 10-32 threads. The breaker is designed for panel mounting and mounts in a 3/4 x 1 7/16in rectangular hole. Airpax manufactures an identical breaker designated UPL1. Carlile and some others also manufacture the breaker but I don't remember the type.
They can be purchased from an electronics supply house as a Heinemann type AM1 BREAKER OR EQUAL. You need to know the trip amperage. The amperage trip-time curve is not critical for this use. Breaker size can be found on either the trip handle or on the side of the breaker.
1986 GB-42 Classic