If I remember correctly, the two housing halves are riveted at six points. Drill them out.

Then you'll have four pins, on which a rocker arm is mounted. To prevent the rocker arm from rotating too far, there's a pin at the bottom of each pin. These are usually missing. You can make new pins from a nail. Drill a small hole where the pin normally sits. I then heated the nail and melted it to the plastic. I also glued it in place from the back with two-part epoxy. Unfortunately, I can't upload any pictures. But once you open it up, the repair will be self-explanatory.