If you are able to remove the hex output shaft pressed into the ball shifter shaft and replace it with your own, you could just add some pulleys/sprockets and a couple standoffs like the attached picture. The picture shown has 30 tooth pulleys and the clearance with the 4 standoffs is close... to get more clearance with smaller pulleys/sprockets you could rotate the entire gearbox slightly about the output shaft and use the 2 standoffs in the upper left/lower right corners (2nd picture). Would be good to work in some more support for the cantilevered gearbox.
