Morse has a friction kit available for the throttle lever. It consists of a plate with ridges that attaches to the side of the cover. A plastic dowel, spring and an adjustment screw with knob that fits into the screw hole hown in #3 above.
The plastic dowel rides on the grooved plate creating friction and the amount of friction adjusted with the knob.
I'm not sure if it still available. Last one I bought 2as 10 to 15 years ago.
Contact Morse/Teleflex to see if it's still available.