First off, the only difference between a 120mm and 126mm loose bearing freewheel hub is that the 126mm hub has an additional 6mm of spacers on the freewheel side and a correspondingly longer axle. You can buy a longer axle and 6mm worth of spacers, and convert your hub from 120mm to 126mm spacing. However, a 126mm hub is more likely to have axle bending and failure problems than a 120mm hub. I would not recommend using this configuration if you are heavier than 200 lbs or place heavy loads on the rear of your bike. Instead, better get a 126mm freehub/casette. You will have the additional advantage that you can run 7-speeds on the casette.