The amount of foam built into the rear tank by the original builder is quite a modest amount. There is no reason why you shouldn’t store stuff in there when cruising – it’s a good way to keep food cool, amongst other things. It sounds as if someone who only raced or day-sailed the boat has added extra foam as a sort of insurance policy in case of hatch failure.
The front buoyancy tank is less accessible, and as you have the shelf on the Mk2 you may decide not to use the front tank for gear at all, so perhaps that would be a good place to add any additional buoyancy – sometimes people put old 5 litre plastic bottles in there, which would add almost no weight – but there’s probably no need to worry.