Why don't you DIY, it's a simple undertaking. Search in youtube, you'll see how it is done. You need 15mm (thin wall) socket, 15mm bicycle wrench, and a simple puller. The chainring and crank arms come in a set. Bring your bike to the part store to make sure the parts would fit the BB axle on your bike. You can also search the net for the part such as in Taiwan: class.ruten.com.tw

There are steel rings (gear) and 7075 aluminum alloy gears, 48-tooth, 52T, and two and three-gear combinations such as 52-42-30. If your Dahon has single mid-gear and you want to put on a two or three-gear crank, you would have to buy shifting mechanism.