I modded a Yeah (rebadged licensed Dahon Helios) from 7 to 9 speed.

I run a 11-34 in the back on XT hub. You don't need a long cage derailer if you have a single chainring. You must have a chain guard because the chain jumps off easily. It works really great. It puzzles me why Dahon do not move to 9 speed on all their 8 speed bikes; it's not really more expensive.