I agree with Duane that you're best bet is to take it to an upholsterer. I'm currently employed as a marine upholsterer, and these cushions would be easy enough for us to do.
I remade my own 13' Scamp cushions a few year ago (and have duplicate patterns for them). I highly recommend that you get new foam (Scamp uses the cheapest stuff out there) and have the company cut the foam 1" shorter along the outside curved/back edge. Scamp cuts theirs too tight which encourages condensation, black mold and makes it difficult to make up the linens.
The best tip for the upholsterer is just to follow the seat dimension on the seat, on the inside of the fiberglass U-channel for a pattern. Don't try to measure up or widen it from there. Easy Cheesy.
Since I full-time, I don't turn the back into a dinette. I have 3 cushions; a center rectangle cushion and the two curved end pieces. I used HyperVent under the cushions (looks like Raman noodles) for breathability, and a 2" foam topper so no seams are felt in the separate cushions.
You may want to still convert that space into a dinette, but I still recommend keeping your foam looser than Scamp does on your new threads.
Good luck and pick a great fabric! I had fun with Tommy Bahama "Southern Constellation"