According to facebook there is since a short while something new on the market:
https://www.facebook.com/pages/STRiDa-up-grade-kit/338168192994991?hc_location=stream
The ATS speed drive kit for Strida!
http://www.ats-speeddrive.com/index.html
Availability and price for Europe unknown so far…
Surprisingly was the new beltwheel already seen before at the Eurobike 2013,
on this - very special! - Strida, shown at the haberstock mobility booth.
This bike belongs to Mr. Schlumpf himself!!! 
(It was lent to Haberstock because they didn’t have a “platform” to present the new 80 teeth beltwheel.)
