I recently acquired a 1983 Trek 613. It has an Atom rear hub with 6-speed Suntour freewheel. Rear spacing is 126. I am wanting to replace the freewheel with more desirable gearing. Would this hub be compatible with a Sachs freewheel?. What do I need to be concerned with on the freewheel with regard to compatibility with the Atom hub and what brands should be compatible? This could be either 6 or 7 speed. Any information and recommendations would be greatly appreciated. Thanks!