Just when you think you've made a decision...
Got a lead on a place that has some good size drops of steel plate. If it pans out I could get enough to fab a keep for about $250. Cheap enough that its not worth risking $80-100 in specialized rods and materials to fix the old one. Wonder what cast iron scrap is bringing at the yard? If it was decent material I'd save it for making stuff but given the voids its not good for much besides ballast!
Sand blast it. Grind out 6X2X1/4 to take 6x2 flat bar across the brake, drill and tap the flat bar on so it is flush with the out side. epoxy the brake together strap and bolt. Finish by glassing so that the original keel sits in a thick fiber glass pocket the metal will shrug off the fiber glass eventually but you are just using it as a mail mould for the fiberglass lay up the cast won't corrode much as their would be no air in the pocket