easy. go on ebay, buy a complete rear wheel, with sprocket and disk included. go elsewhere on ebay, and purchase complete rear brake setup, and brake lever. find a welder, a drill, and then when the parts arrive, throw them on, and make a bracket to mount the master cylinder out of flat bar/plate... drill holes to mount said cylinder...bolt on, pump brakes to see they work...
damn sight easier than trying to re-hub your existing rear wheel and lacing up spokes etc etc...then trying to get disk components to fit...