Gotta love galvanic corrosion....
A couple thoughts -
First, do as mmiller suggests and try to get the bolts free. I guess you could throw an 18" breaker bar on them and see if you can get them to move. But you might be better off cutting in the inspection ports first so you have access inside the hull. This would allow you to 1) see what you're dealing with before you start breaking stuff. And 2) apply penetrating oil directly into the threads. More than likely though, the threads are caked full of corrosion and no amount of oil is going to make a difference.
If you do break off the bolt heads, I think it's not the end of the world. Basically, you would need to take an angle grinder and cut the bolt off flush with the deck and then drill it out. If you end up destroying the threads during the bolt removal, you would just need to drill and tap the aluminum bar in the hull for a helicoil or thru-bolt (which would be no problem since you have access ports).
If things really go south, I imagine you could cut the aluminum bar out of the hull and glass in a new bar. Again, this is why you probably want to cut the access ports first, so you can formulate a game plan.
Another thought would be to repair the crossbar in place. From the picture, this looks like a fairly clean break and I'm thinking you could unscrew the mast step ball and remove the nut and washer. Then repair the crossbar using a plate of ~.100" thick aluminum alloy 6061-T6. The plate would be approximately 5" long x whatever the width of the top of the crossbar is (4" or so?). The plate would need to be bent to conform to the curve on the front of the crossbar. Then drill a 3/4" (?) diameter hole for the dolphin striker post to pass through. Re-install the washer and nut on the striker post and then match drill 3/16" holes through the plate & crossbar and secure the plate in place using SS rivets.
Whatever you do, if you end up removing and replacing the bolts, be sure to put some antiseize on them this time.
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