This link, is for replacing the sensors on an oilhead bike .
But it shows how to make a tool for attaching the sensor to the plate, I think this would work just as well on an airhead ignition canister, aka ' bean can ' .
Instructions for making the tool, are on page 9 of the link .
http://advwisdom.hogranch.com/Wisdom/oilhead_hall_sensors.pdfIf you do have an oilhead, the complete instructions are there to accomplish this task .