Can any, some, all of the MU's have the front chainring changed for one with fewer teeth without replacing the whole crank assembly?
Do they take any special chainrings or just anything with a matching bolt diameter?
Would the bolt diameter be standard?

Just want to know before pulling the trigger and finding a new bike.

The short answer is, 'yes'. I can heartily endorse these guys for the parts you want. http://www.gaerlan.com/dahon/gear.htm

Yes you can even add an inner chainring! Although there is no commercial front derailleur avalable here yet, there should be soon. You can shift by hand, if you just need an occasional dual system. I added a 38t inner chain ring; giving me a 22 gear inch low. I can almost climb a wall!

Thanks,