My sunfish got to be over two hundred lbs.I opened up the forward deck to find only expanding foam. No Styrofoam. I see pics of the Styrofoam but need the specs/dimensions of the Styrofoam. I have been drying the foam (slow process) but Styrofoam would provide better balance
The best bet on a job like this is to stick a tape measure across it and see what you've got. While technically a "one design" hull a modern sunfish is a long way from a classic one and a wooden hull may have changed shape over the years.
Styrofoam and the like comes in standard sizes so if you're doing the job yourself you'll be buying blocks and carving them down. You should also do a full check of the hull integrity while you've got the top off as it sounds like you have a persistent leak to get into this situation.