Get a Teleflex CC640xx series cable. (If it were an 18 ft cable, the number would be "CC64018" as the last 2 digits are the length in ft)
This is a heavy duty push-pull cable. It has 5/16x 24 ends, with 4" of travel. One end of it has a bulkhead fitting with 2 jam nuts. The other end is designed to fit in a sleeve, and a setscrew fits the groove near the end of the cable sleeve to hold it in place. You'll need to either drill out and re-tap your existing ball joints, or buy some that are already set up for 5/16x 24 cable (I think seadoo jet boat steering ball ends are like this)
Stick steering would be fairly easy. Just a lever with a pivot point at the bottom, and a few inches up from that, would be your attachment point for the ball joint and cable. Then a bracket to hold the cable itself.