"If I force it, it will stay over but you can hear a hard internal knocking and probably damage happening."
How far over does the knocking begin? Only at the extreme? Or does it knock all the time under load?
With everything in proper condition there should be three definite spring-detent positions for the pto shift handle - low - neutral and high. What I'm suggesting is once the lever feels like it is in either the low or high speed notch, and the guide plate is not influencing its position, find a way to hold the lever there so it can't move. Then try the implement and see if it stays in gear. If under those conditions you hear knocking then you likely have broken pto gears in there.
One possible cause of popping out of gear is pto gears that are worn enough that they tend to push themselves apart along the axis. As gears wear the teeth can become slightly sloped - especially if they have run partially engaged for a long time. If it isn't too severe, the disengaging force can be overcome with a positive restraint on the shift linkage. It's possible to run fine for many more years that way.