This contraption is a speed controller I made from a Dick Smith voltage regulator kit (almost before you could buy any modern ESCs). Servo turns the pot and micro switch turns power off in neutral (stopped) Reverse was a toggle switch operated by a servo (as described previously in the 'cheap reversing switch' post.
The other pic is of ready made version I just bought (Ali Express) which is more powerful, and can also be used as a speed controller/voltage reg for anything. Could be operated by a servo on its side with a flexible coupling. $3.55 NZD cheap as!
JB