I would think that as long as the luff measurement is pretty close to the pinhead 5.5 main, you could run any Nacra F18 squaretop on it. There should be a few used ones around for cheap. Then, all you would have to do is have a good sailmaker install the outhaul from your old boomless main to the new one, or just get a plate with several holes installed, and get a hook for the top of your mainsheet blocks.