Considering the amount of effort it will take,why not just build your own frame modeled after a Brompton? A cheap bike will prolly be crappy hi-ten,if you start fresh you can use much better steel.
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C'dale BBU('05 and '09)/Super Six/Hooligan8and 3,Kona Dew Deluxe,Novara Buzz/Safari,Surly Big Dummy,Marin Pt Reyes,Giant Defy 1,Schwinn DBX SuperSport,Dahon Speed Pro TT,Brompton S6L/S2E-X