Re: any ideas how to make 1/5 turn throttle?
You could make a reverse "snatch block" system. I'll describe the cables and you can fill in with your imagination what it would take to fab a bracket to hold it all. This would reduce the standard rotation of the twist grip to half of normal but make the throttle twice as hard to pull.
Devise a way to attach a small ball bearing pulley with a bracket that the twistgrip cable would attach to above the pulley.
Then bring the carb cable from below, around and over the pulley, with the end attached to something solid and unmoving.
Now, when the upper cable is pulled, the free end of the lower cable will travel twice the distance that the upper cable moved.
I've done the same thing but reversed, to the brakes on an ultralight to give more power to the brake arm for stronger brakes.