Sounds like you aren't drawing a ton of amperage, so there may be some easy options.

The two that come to mind are:
-Double pole double throw switch (DPDT), connected to a servo & on a 3rd channel.
-The other would be a double pole double throw relay & use something as the controller such as an old servos motor output or other wise.

RadioShack switches that should work are:
Catalog #: 275-652
Catalog #: 275-636
Catalog #: 275-663

Drill a hole through the switch lever add a linkage some collars & possibly some pen springs.

The wiring will look odd, but the motor or ESC can go to the "common" on the switch, & the other will go on the outside poles with a crossed jumper to the other set of poles.

If you are only allowed one servo total, The switch can be controlled by steering too far to the left or right, but it might be tricky to drive.