I'll have to look tomorrow to see how the top roller is mounted, the one that moves. One side uses an air cylinder which moves a cam, which has a pin in it to rock the roller back and forth. An optical sensor actuates the cylinder when the belt passes in front of it. So basically it's adjusted so the belt moves toward the sensor, and when the sensor activates the cylinder, it causes the belt to move in the other direction. Nothing complicated, just a sensor which switches a cylinder on and off. It's probably a double acting cylinder.
You'll need a pretty large motor I think. The one on ours is probably around 10 HP.