That's easy to do with one of the square Bosch-type relays. Hide a momentary-contact push button somewhere under the dash. Use it to energize the relay in the circuit from ignition switch to coil that needs to be made to make the engine start and self-holds until the key is switched off. Once the engine is stopped, you'll need to push the switch after turning the ignition on. It's a simple self-holding circuit we used to use on industrial machinery.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!