From the rules:
34.4 Positive buoyancy units of closed cell plastics foam. Shall be securely fixed within the hull of G.R.P. and composite boats, as follows (lift refers to buoyancy when submerged in fresh water):
(a) (MK I and MKIA only) One unit providing not less than 81.65 (180 lbs) lift in the forward compartment. Not more than two units providing not less than 40.82 (90 lbs) total lift in the aft compartment.
In plain English: anything buoyant with a total volume of about 40 liters will do. Since the new boats (World, MK-IV) use plastic cubitainers which are very similar to thin PET bottles without a cap, I see no reason why you can’t replace the foam with bottles or kegs. (Years ago we believed that bottles were unsafe because they could spring a leak).I like those red capped plastic Sauerkraut kegs that have the extra benefit of providing extreme dry storage for my valuables and sleeping bag. I got mine from a pet shop where they were sold to store pet food.
There are some catches though:
1. The buoyant material serves no purpose as long as the compartment is waterproof but they are the only thing preventing your boat from sinking should the compartment ever flood. There is no way to bail the boat once the compartments are flooded. The extra buoyancy only prevents the boat from sinking, no more. The extra buoyancy is important, for your crew’s safety.
2. If, in a worst case scenario you should loose your hatch, the buoyancy material should not be allowed to escape. In other words it should be fixed inside the tank. And do not underestimate the buoyant forces involved when 40 liters are submerged.
Back to the motor bracket, I would simply remove the material, fix the bracket and figure a way to fix any sort of buoyant material again. Think of it this way; you finally get a chance to make the aft tank more efficient in terms of storage space.
AFAIK an American Wayfarer glassed in some empty Super sized Coke bottles (PET bottles). For extra security you could glue the caps on. From a measurer’s view I don’t care what one uses as long as there is 40 liters lift. A nice solution are two 20 liter plastic kegs fixed to the hull with parachute ribbons and spanners. That secures them in place but you can still remove them and use them for dry storage. A less sophisticated solution would be to just tie them to the boat in such a way that, should they ever escape from the compartment, they would still pull the boat up. For this you could screw some deck clips to the hog inside the tank. Simply gluing blocks of styrene directly to the GRP isn’t strong enough, you need to glass them in again.