How are you going to change cog sizes with a disc brake in the rear? Unless you do the double crankset with dingle cog or dos freewheel, your axle will sit in a different location in the dropout and the rotor will be partially out or too far into the caliper. The Genesis Flyer also has true horizontal track fork ends, which won't even allow a rim brake pad to follow the rim and maintain ability to brake after moving the wheel to change gears. Removing the rotor and running fixed would solve this but then you have the difficulty of downhills without stopping at the top to change to a higher gear. Aside from having custom plate forward -opening horizontal dropouts brazed in or having a custom frame built, the best way to get this done is to find a frame that fits with Campy 1010a, Huret, or similar long dropouts, then build around that.
Phil