what i've done is find an old steel youth racing frame designed for old 24" wheels (540mm ISO) and chuck 650c wheels (571mm ISO) on it, as well as putting 155mm cranks on (Miche Young). changing the wheels also means you can put a regular drop brake on (after drilling out the back of the fork to 8mm).
these kind of frames turn up really rarely though, although when they do they're usually pretty cheap.
what i've done is find an old steel youth racing frame designed for old 24" wheels (540mm ISO) and chuck 650c wheels (571mm ISO) on it, as well as putting 155mm cranks on (Miche Young). changing the wheels also means you can put a regular drop brake on (after drilling out the back of the fork to 8mm).
these kind of frames turn up really rarely though, although when they do they're usually pretty cheap.