You could probabaly find a freewheel with a 34 tooth largest ring. it ought to be a six speed or it may not fit in your frame. You may also need a megarange derailleur. You could also drop a few teeth on your front sprockets. You have to find sprockets with the same bolt circle diameter (BCD) and not let the difference between big and small rings get any higher than what you have now or your front derailleur won't work.