Speed Drive, is the other option, with a 34t chainring, the 1.6x over drive gear is like the 54t chainring.

their mounting is a bit different, the BB shell gets chamfered edges.
a compressible aluminum conic washer grips the 2 surfaces,and the BB fixing compresses both ends..

I got a MD from this guy , but several years ago
http://foldabikes.com/PriceListHTML/...ountdrive.html

Im using the 54:15 gearing, though the 13 or 14t could be used . the chainring BCD is 130
so other chainrings are possible like a Surly Stainless steel 50t.
The heel and Bar lever double shift works really well.. and Quick, enough,

your contact may be correct about discontinuation, by the Swiss Company as the CHF stays strong
Euro and dollar need more to buy them as they lose value**. ( harder for CH manufacturing.)
they are said to be shifting making them to Germany for this reason.

**[& the plutocracy moves their money to Swiss numbered accounts]

a further thought , the standard MD uses a coaster-brake like strap
around the chain stay, the Brompton version uses a Button, that sits on top
of the piece where the rear Hinge pivot axle is, behind the BB& seat post,
perhaps the 2 small parts can be ordered and exchanged for the standard type's parts.

Or, If you put a sturdy metal Knob in the bolt hole , for the band strap
in the standard torque arm, that would work..

http://www.schlumpf.ch/hp/schlumpf/schlumpf_engl.htm


used Sheldon's gear calculator:

Brompton . MD 54t, / '21.6t'



hub cog on Aw3 ... x .75,1,1.33
L; N; H


13t
50.1/20.0; 66.8/26.7; 89.0/35.6


14t
46.5 18.6; 62.0/ 24.8; 82.6/33.1


15t
43.4/17.4; 57.9/23.1; 77.1/30.8