|Yes the leak is common and yes it can be fixed by removing the cover plate on deck, cleaning out the existing bedding compound and replacing it. If the bedding is original, it's butyl tape, which many people prefer as it doesn't harden.
The area should be monitored, though, as the loads from the mast tend to move the chainplates constantly.
The deck surrounding the chainplates on this boat should be checked carefully for water entry and delamination as the water that rotted the bulkhead could easily have also entered the balsa of the deck laminate. It's messy but not a huge job to pull all the chainplates, clean the area, then scoop out the balsa at the edges of the chainplate holes and replace that balsa with thickened epoxy.
C&C 27 Mk IV