I refaced my BB down to 68 (I think i took the sides down slightly asymmetrically so that my chain line would end up nice and straight) then used a 24TPI plastic cupped cartridge unit.
I think I used a *** or Kenitx unit for a brompton unit either 113mm or 119mm long (fairly cheap).
I had to remove a little of the thread on each cup (I used a lathe but a file would work) to allow the cups to screw down to their shoulders rather than bottoming on the thread.
It's worked a treat.
Good luck!