how many rear sprockets to you have? If it is 6 or 7, you should definitely be able to find a freewheel like this one, which has a nice 34t bailout gear that will help you out, and it is super cheap. Chance are that shifting will be much smoother with the hyperglide freewheel, too.

http://www.amazon.com/Shimano-MF-TZ3...9287470&sr=1-2