If you do something like this the switch type is critical. Do NOT use a toggle! If is is accidentally bumped, or you hit a pot hole it will actuate the starter while you are riding. Always use a button such as a dirt bike kill switch. These push to actuate and release when you stop pushing.
DO NOT USE ANY TYPE OF SWITCH THAT DOES NOT AUTOMATICALLY RELEASE!!!
FYI: When I find a wiring harness issue, I usually simply bypass the harness and run new wires completely. The switch should not be that hard to check. Pull the headlight, unplug the switch, and pull the switch loom out.