It appears you are also missing the parts needed for the x-axis. They would be similar to what you've got for the y-axis there (some sort of turntable bearing, pipe to hold centering springs and to house wire, another set of gears and a pot, and any mounting hardware).
Also don't forget you'll need something to interface this to the PC. If you use 100K pots this could be hooked to a standard joystick port connector. Or you could use 5K arcade pots and hook it to a hacked Microsoft sidewinder joystick (USB).