Without accurate data on the current configuration of your hull it would be very difficult to give advice on how to modify it. If you could post some photos and take as many measurements as you can of the hull, keel and any other bits you may have then I'll do my best to help. In the meantime, you might consider these points...
A hull of this sort will not be happy with a lot of added weight. The best solution I feel would be to create a 2 man daysailer, decked over with perhaps 6mm marine plywood with an appropriate cockpit cutout. You could even try 3mm ply with an epoxied down laid teak deck of 3mm if your woodworking skills are up to it. You could investigate scale factors to scale the rig etc. directly from proven 12 metre designs, plugging your values into the rule formula, or start from scratch and design a rig which suits your sailing style and conditions. The most important thing would be to check things like displacement, Longitudinal centre of buoyancy (shouldnt change much with heel as the boat is narrow), stability, and centre of lateral resistance. These things will tell you how much sail the boat will carry and where to put the mast for balance.
With a boat of this size that is being used in calm inshore conditions, you could almost get away with a bit of informed guess work (not particularly recommended) but if you could post some photos and perhaps the lines plan (if you have it) then that would help a lot, and certainly an accurate displacement measurement would also be of much use.
Incidentally, where did you get your hull from?