the 130 BCD will take any single speed type chainring.. (omitting the shift ramps , etc.) you can order it with the 50t. of course the chain ring is included

Mine came stock with a 54t , so I use a 15 t hub cog.

see E.1.6.1. Brompton - torque lever http://www.schlumpf.ch/hp/schlumpf/faq.getriebe.engl.htm#C

the speed drive , is an overdrive ... with a 34t ring , kicked into High it acts like a 54t.

[site said with 110 spider option, the standard Chainring has to be made with more clearance around the gear case,
so fitting say a stainless Surly 34t you need to do a little file work ]