First, get rid of the old speed control, that uses resistors to control the current flow. That old dumas speed control will heat up more when the power is in the low position and drain your battery, and get very hot.
If you put in a 40$ Proboat ESC which comes with a battery elimanition circut and has a reverse,12 volts is its limit ant it will handel that motor with ease(I am sure there are many other good ESCs) This is how you would set it up. 2 wires go from the battery to the esc, do not reverse them or it will burn up the esc, two wires will go from the esc to the motor, if you reverse these wires, it will only make the motor run in reverse. of the 2 wires left on the esc, one goes to the reciever, that is the top box in your picture, You would replace the center wire.
Becaues the proboat esc has a battery elimnation circut you would not need the wire that is plugged into the right side of the reciever. Unplug and remove it and the small pack of batterys that only give power to the esc, the new esc will pull power off main battery to run the reciever. The only wire left on the new esc is the on off switch, always turn it on after you turn on the transmitter and turn it off before you turn tne transmitter off. you dont want the esc on when the reciever is not on or the esc and survos go wacky.
This will allow you to get rid of that survo on the right of your picture, the large silver esc and the battery pack the runs the reciever.