The plug you can usually get at any Lawn Mower shop as well as some Home Centers such as Home Depot and Lowes.

The kill switch can be any kind of switch, slide or toggle (radio shack can help you out) that's SPST or DPST.....
Just hook the kill switch between the engine block and the empty terminal on the magneto. The engine is "killed when the switch is closed, i.e. shorting the magneto connection to the engine block.