N+1. It's really the best way. Performer still designs and builds USS bikes. Buy one. Or find one used. A used Performer will be cheaper than a new USS kit (or the parts) for a Challenge bike of a certain age. But if you really want to wrench on the Challenge you are on your own. There are no USS kits that would work. Linear might have something but nothing you buy will work without major modification so you may as well just study some examples of what you want and hit up the ACE Hardware and/or McMaster Carr and fabricate something. But I doubt it will look and work as well as a bike designed with USS in mind from go.