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I also have a 1987 Montauk. The previous poster is correct the original circuit breaker is a 30 amp. You can find them on amazon. I just replaced one with this same part. if you search these words on amazon you will find it. They also sell the rubber clear boot that covers the push button to provide some better protection from water.
New Boat Circuit Breaker Push Button 30 AMP Marine AC / DC BLU 7059
Mark