over here, the bikes with ignition keys were mainly electric start models
if you do add one to your bike, i'd suggest you solder the wiring into your loom,
instead of using the plastic plug/socket, as all you need to do to bypass it is to unplug it from your loom and the bike will start.
i guess you could always measure what wire does what in the ignition switch with a multimeter, and modify your wiring a little
then it will only start when the ignition key is turned on.