The side lever on that model only does the stop function, nothing to do with the RPM's.
It has 2 controls that run it one manual (rod) and one with the Stop solenoid which is timed to engage for approximately 5 secs.
Pull the plate off and either take it apart and free up the shaft or replace the spring in it to make it work properly again.
You can't simply tie it open or closed as you will not be able to either start the tractor or shut the tractor down.