keyless ignition?

I bet you could put something together pretty easy, but it might not be the most efficient setup. From an electrical standpoint you could basically just hook up a push button start switch like a race car and all you would need is a relay that cuts the push button off unless a transmitter is in range. Then all you would need is some RFID or similar transmitter that would engage the relay whenever it gets within a certain range. That setup would constantly use power though since the relay that allows the push button start to engage would always be active. Probably wouldn't cost much at all though, just need to look up a transmitter kit.