There's not a lot to one. A machine screw or small bolt with a stack of nuts jammed together on it for the extension should work just fine. The extension goes toward the rear of the tractor and is put in front of the cleat made on the throttle rod. Front from looking at it at the pony motor controls.
When you pull the throttle out the cleat contacts the pin extension and restrains the governor arm, pulling the throttle butterfly closed. Push throttle knob in and the governor has more room to open throttle butterfly to bring rpm up to governed high idle and then respond to load increase.