Mudlark, my 1987 23-2 has original cabin cushions. The foam was full of mildew but the covers were salvageable and cleaned up nicely after washing them a few times. I got quotes on foam from local sources and the Internet but 4 inch thick foam is pretty pricey. I kept looking and found 4" thick foam mattresses on sale at Overstock.com. I bought a Full size and Queen size for a total of $150. I used a $15 Black & Decker electric knife to cut the foam to size using the covers as a template. The mattresses are an inch or two shorter than the old foam, but I'm guessing we'll never know the difference.
Sounds like a really good buy. If you have pieces of foam left over you can us a spray adhesive to join the small end pieces to your new cushions to make them full size. The adhesive is available from many sources but I know Sail Rite has the adhesive. A comfortable boat cushion is all about the density. For sleeping we have use high density foam so we don't bottom out.
Tom L.