I'd keep the rules simple. The bikes would be required to maintain a mostly stock appearance, meaning no fairings unless they were factory original or a listed option, and I'd want them to at least look like cruisers, so they'd have to carry front and rear fenders and side panels. Engine mods would be permitted so long as the stock bore, stroke and cylinder-head castings were used. Suspension, brake and wheel modifications would be allowed, but changes in the frame geometry would be prohibited, and that would include relocating the rear shocks. However, I would permit the relocation of things like the bars (no clip-ons, though), pegs and controls both for safety reasons and to improve comfort and control. The bikes would also have to meet a minimum weight limit.