That panel looks pretty dated. I wouldn't presume to tell you how to spend your boat bucks, but you could install a modern Blue Seas 8100 ELCI 30-amp main panel at the shore power inlet where those mains are located without too much modification to the panel - assuming the load breakers are good still. I just did the same... Only cost about $200.
Or for a little more you could replace the whole main breaker and load breakers with or without an ELCI for just a little bit more. It looks like the wide non-ELCI 8099 panel could fit where the load and main breakers are in your picture. Most of the panels come loaded with breakers.
Both options are a bit more expensive than finding the odd breaker, but give you a more updated looking and safer setup.