I think you could do something pretty cool starting with a DR-Z400SM - they're cheap, reliable, single-cylinder with lots of aftermarket support, and it comes with a pair of 17" laced wheels.
A kit that shortens the forks, replaces the triples (to correct rake & trail), provides clip-ons or clubmans, re-springs the rear end, and replaces the tank & seat would be doable to manufacture and fit, and the result would be a pretty cool machine.