It really is an example of a place where they choose to change the design just a hair and it results that things are completely frustrating when trying to use off the shelf parts. Are there BMX bike forks that are 1 1/8"? I don't know much about them - try asking Gaerlan for ideas or looking through the Better Bicycle catalog at places like aebike.com.
The same thing is frustrating with the rear dropout spacing. They choose sizes between 118mm and 126mm for a number of their bikes. Adding 14mm (7 on each side) is barely going to affect the folding size, but would easily allow putting a Nexus or Rohloff rear hub in one of those, or an off the shelf road or mountain hub. Why do they do that? There are some other frustrating examples of incompatibilities they bring in, like the SDG IBeam saddles on some of their new bikes. Dahon has such a commanding lead on their stuff they can do what they want. And the replacement parts aren't really overpriced relative to other cycling stuff.