There's a new kit from BMSBattry that would work on a folding bike, but I wouldn't use that Dahon as a donor because its brakes are insufficient and its gears are over-specified. I'd get something like this one, which has disc brakes, freewheel gears and is a lot cheaper.
http://pages.ebay.com/link/?nav=item.view&id=281761036750&alt=web
A 201 rpm Q100H in the back wheel and this battery/controller kit and you're home and dry:
https://bmsbattery.com/ebike-kit/633-q100h-36v350w-rear-e-bike-motor-wheel-ebike-kit.html?search_query=Q100h&results=5
That's weird. The batteries have gone. A few days ago, I saw on their website black plastic rack batteries with a rack with an included 20A sinewave controller, like the 09 bottle battery kit, but as a rack battery. It's not there now. Did anybody else see it or was I dreaming?