Once a disc and caliper is chosen, it's mostly a matter of bracketing the caliper to something rigid enough to carry the torque when the brakes are applied. Fortunately, the bolt holes for the drum brake backing plate are right there to work with. When I used to fabricate brake systems from scratch on circle track race cars, I would use air pressure to clamp the caliper onto the rotor where I wanted it to fit, and then fabricate brackets to mount the caliper firmly to the spindle from appropriate thickness steel plate. Most calipers need to be able to slide side to side to self-center and to accommodate brake pad wear, so be sure to check clearances closely to avoid interference as the caliper moves in and out. A plasma cutter and some sheet metal templates makes the fabrication job a lot easier! Good luck!
Jerry
