Vandeusen, my Clipper 34 had soft decks, and the PO had addressed this by just removing the teak, then having a boatyard lay down marine ply over the top of the top layer of the GRP sandwich the decks are made of. They then fibreglassed over this ply, finishing with a non-skid surface.
The damp core was then trapped between the upper and inner layers, and rendered effectively harmless.
it worked well.
However, whatever you do, as long as you seal it enough to prevent any further water ingress, it will be about all that is worth doing for this age of boat, and the price you paid. A full rip-out of the damp core, which is a huge job, requiring removal of almost all of the top GRP layer as well, unless you do it all yoiurself, is just not justifiable in terms of cost, and the little it will affect re-sale later on.
If you are planning to leave the teak in place, then that will be enough stiffening if you then encapsulate it in a waterproof layer, then finish off with something non-slip, like the truck deck stuff someone else suggested. So I doubt you need to mess about with a foam. But whatever you do make sure it is watertight. Then even if there is a bit of spring in places - you will never go through it..!
