Any device to interrupt the current to the starter circuit, coil circuit, or
electric fuel pump circuit is what you want. Interrupting all of these circuits
independently would be ideal. It would be next to impossible that a thief
would easily find all of the devices and defeat them in a sort period of time.
Using a momentary switch, magnetic reed switch, normally open or normally
closed switches, an RPM activated switch in any number of obvious and hidden
locations in each of the above mentioned circuits is only limited by your
Consider wiring in a simple pizo noise maker chirper thing to attract attention.
Heck, it's not rocket science, right Steve?