I used to sail Ray Greene Rebels but don't know how a Rascal is constructed so what I might tell you could be wrong.
What you are saying is that the cockpit
sole is the inside layer of the hull and that the hull has a balsa core
In that case I'd grind that cockpit sole, remove the soggy balsa and epoxy
resin and glass over a couple layers of mat and glass where the balsa core was, i. e. 1 mat, 1 cloth, one more mat and finish with cloth. Once that's done I'd put a thin layer of plywood
cut to fit but not necessarily glassed in. That ply is what you would walk on in the cockpit. If The hull is too flexible then I'd add another layer of mat and glass. I think that would be strong enough and keep the weight down.
Usually a balsa core was to add rigidity to a sandwich of layers of glass to keep weight to a minimum. Removing the balsa will make the hull flex more but adding double the thickness of the glass that you removed to get at the soggy balsa core should stiffen up up again. You could glass in some cardboard tubes to make light stringers and ribs but I don't know if you want to get that complicated.
Good luck with your repair.
I hope others here in the forum will give some guidance as well.