Only one way to fix that...
Remove the cleat (it will be a ridiculously long bolt) grind the rotten stuff off completely with a #24 grit grinder until you remove the rotten loose stuff, then start building back up with fiberglass. That looks like a wooden round block in the picture??? You might want to make another block out of 1/2 or 3/4 plywood, cut to shape and glass it in.
To save time on colour match just paint a strip up the centre of the top of the hull. It will look new and save on colour matching.