Any auto parts store should have cheap, universal "keyless entry" systems for cars. I think the brand I bought and installed in my Subaru is "Bulldog", and it cost 30 bucks. The receiver and relays could easily be adapted for use on a motorcycle as an ignition circuit cut-off. In fact some of them come with a relay specifically for that purpose, where the ignition circuit is only capable of being turned on when the system is disarmed.