Even the lightest foams (about 30kg/m^3) will add about 25kg to your platform from my estimates. I think the epoxy expanding foams weigh in the order of 300-800kg/m^3, which would add about 240-640kg to your platform!!! That will probably be close to sinking the boat rather than making it unsinkable. It certainly wouldn't sail on one hull...

If the deck is parting from the hull, cut it off and inspect from the inside. You'll get a better idea of the work that's needed and you'll be able to glue the deck back down properly later if you decided it's a feasible project. It could be a very good investment of an hour or so cutting the deck off, even if just to see how it was built and what went wrong.

The sounds of it though are that it should be dumped. If it's single skin it will be easier to fix than if it is cored.