Could you provide any more details? I have no idea what is the needed standard for the cogs - have not looked there ever yet. What spacers are you using? What shifter? A thinner chain? Thanks.
It is not much rocket science involved. I used cogs from a shimano cassette (in my case a Shimano CS-HG500-10/11-32) and the stock spacer from the 6-speed. The small 12-teeth cog has a built in spacer. You have to dremel on the 12-teeth a bit to make the ring that keeps the cogs in place fit. I'd recommend to use a donor-cassette with the smallest cog being smaller than 12 as the smallest cog will look a bit different and will probably not survive for long when being dremeled to the necessary amount. Any shimano-derailleur cog will physicaly fit the BWR-freewheel so you have quite a bit of choice. The setup with 12-14-16 on the BWR in combination with the 44 chainwheel offered the best shifting steps for my taste but your mileage may vary.
The derailleur has to go a tiny bit further outside so i cut an old credit-card to form (2 layers are perfect) to fit into the derailleur on top of the first axle nut and act as a spacer - my very own form of credit-card-supported touring.
Shifterwise I am using an S/A SL-S30 3-speed thumbshifter on the right and a optically identical Sunrace M90 friction thumb-shifter on the left. I like these better than the stock shifters - but you can use those too: 3-speed hub shifter obviously does not need to be touched, 2-speed needs a 3rd position in the middle to be cut at the inside part of the shifter - you have to figure out the correct postition for this.
To make the shifting work you have to adjust the little screws on the derailleur that limit it's end position as you would with any derailleur.
And I am running a 10-speed chain as the cogs came from a 10-speed casette. Possibly a 9-speed cassette and/or chain will work as well and safe on cost. So in the end for me cost was ~25 € for the cassette, ~20€ for the chain and ~25€ for the shifters, the latter are not even necessary. The whole mod took about 1,5 hours.
Before and after:
Shifter
inside the original 2-speed shifter, not yet cut a 3rd position
cassette
cogs mounted
You can see the spacer between frame and derailleur in this picture - at this point in time it was not made from a credit card but a perfectly made aluminium spacer that came with the Xrf8w kit i bought from tillercycles in the uk. I had to replace it as I refitted the 8-speed to another Brompton and needed it for that purpose. Also the 12-teeth has been dremeled a bit more intensively in the meantime to make the ring fit more solidly - no trouble at all with the shifting.
