well i dont know if i can help but to switch to a belt drive (i've never seen one on a ironhead but)you would need to change to a disk rear brake.......little bit of work there but it does open up the wheel selection......... one of the ways to do this is to change the rear swingarm to a '79 arm and do some mods from there..........
http://www.sportster.org/tech/disc-conv/ this link will explain..........then from there you can run any wheel for the 79-81