Why not a cheap and easy option, replace the crank with a double one? I have a Flamingo 3 speed and put a double plate, 50 t and 39 t. It solves the problem inmedately.

To go further, sometime ago fietsbob mentioned his schlumpf drive, but that may be expensive.

I considered adapting a derrailleur? for the plates, and got the piece to atach the mechanism, but didnt go on with the project.

I lived in a three flights apartment before and anything heavier or bulkier than the Flamingo or Brompton, to say, was out of my mind.

Now when time came to carry my kid and face very very hilly streets in a mountain town, I turned to a Dahon Speed TR, with a dual drive and 24 gears. Otherwise was impossible to ride. But, at the same hand, I dont need to carry the bike upstairs anymore.

Now to ride again the Flamingo I'm thinking about replacing the 39 t with a 30 or something smaller, the smallest the chain long can handle and replace the rear one with one bigger also.