That's just a very simple single contact relay. Since you're running 12 volts, just go to the corner parts store and buy a Ford-style 12 volt starter switch and wire it up to energize the starter-mounted solenoid. Use the "start" signal from the ignition switch to trigger the Ford switch.
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!