You would be able to start with the Nacra 5.5 platform. The rest will be tricky if you are not a nautical engineer. Boats are designed to be balanced based on sail to board CGs. If you make a smaller jib or larger fathead main you will throw off the balance. I think by the time you get it all right you will be many hours and $s out. Maybe think about Baltic's idea of a used Tiger. They are still very competitive boats in the class.

Later,
Dan