My suggestion:

Grind open the slot in the existing bolt rope groove until it is wide enough to accept a carbon fiber tube big enough to fit an acceptable size bolt rope or slugs through. Then epoxy the tube into the slot with adhesive high density filler + west system epoxy. Overlay the tube with layers of carbon cloth and eopxy spread onto the hull on either side. Then cut a slot in the carbon tube using a dremel and a straight edge jig aligned with the tube. Fair out with low density filler. Then finish with gel coat (which can be applied over an epoxy repair)

Chris.