Have you got battery vottage running to the coil? If so, you have either wired the switch completely wrong, or the wrong type (automotive) switch is fitted. The engine is MAGNETO ignition, which means NO connection to the coil for engine to run, GROUND the coil to stop.
Let me know if you need further help.
phil_saunders(a)bigfoot.com
Phil Saunders, November 2009