Well, setback. The good folks at GoPowerSports were kind enough to send me photos and sizes of the small calipers used on the Chinese quads, go-karts, etc. They would work and fit if I was going to keep the suspension at an off-road level (the A-arms at a slightly negative angle). If I'm hoping to use this on road with street tires, I will not be able to use standard car tires with a flat contact patch (the A-arms at a zero angle / perpendicular). The reason for this once the A-arms are at 0° there is very little clearance between the A-arm and the shock (~1") and, indeed, it appears a PO must have driven the A-arm into a shock at some point since one is bent.
So, my options now are to 1) find a 5" drum brake, similar to ones used on ATV's, and a way to mount them and/or 2) find a 4x4" wheel which will allow me to mount a motorcycle/scooter tire (with a rounded profile) so that I can adjust the suspension to allow clearance for a caliper. Ideas?