How about a combination lock?
get 10 position rotary switches
the more you get, the more complex the lock
wire them so they all have to be at certain numbers in order to provide power to a push button
push the button to start.
You could even have different combos make different parts of the car work.
3-4-2-5-9 turns all power on, lets you start the car
3-4-2-7-3 Just turns on ACC, lets you listen to the radio
You could set it up so that any other combination causes the horn to honk when the button is pushed. =D