One option is to take apart the steering yoke and add a wire ring terminal, (the kind you would use to crimp on the end of a small wire and place over a bolt for a connection), place it around the srew that holds the throttle lever and spring. This will force the end of the wire terminal to hit the back side of the steering yoke when pressed. It will keep the lever from being depressed to full throttle, you can adjust where it hits.