I know nothing about the Brompton set up but I have run 3x2 using the Sachs Torpedo sprockets and spacer on both a Sachs and an SA hub. The sprocket spacer is pretty essential to make the thing work properly. The set up I had was 1/8" and dished sprockets. Having had a look at the Brompton set up in photos it would appear that the 15T sprocket is a standard flat (they all are in that size, dished sprockets start much bigger) sprocket in 3/32 size but with cut outs on some of the teeth (which explains why it has a specific orientation). I know from experience that a sprocket without cut down teeth will happily change (at least in 1/8" models). There may be special features of the operation of a Brompton derailleur which require it to have cut down teeth. That's outside my knowledge or experience!
Which leaves the problem of the 13T sprocket. I would think that this is a Brompton specific cog. If they don't have it any longer chances are it doesn't exist (at least as new). However I can't see why a different cog wouldn't work. The essential part to my mind is that the total of the spacers is the same whether you use the Brompton ones or other "système D" alternatives. I think that ss cassette conversions use the same diameter spacers and, depending on the supplier, are available in a wide range of widths. (Velosolo comes to mind as a supplier of assorted spacers going down to very small - I used them about 10 years ago).
Now over to the Brompton specialists!
