I've wired up a replacement starter button using a DPDT momentary switch (Double Position, Double Terminal, momentary). This means that you can wire up one circuit so that is is permanatly on, and will become momentarily off while the button is down (the headlight circuit) and the other circuit is permanatly off and is momentarily on while the button is down (the starter motor circuit). The electronic equivalent of killing two birds with one stone? You can get these switches from most electronics suppliers, you just have to ask. The other type, SPST is more common, and is used for single circuit applications - like door bell as it only has one 'on' position.
Hope this helps, cheers.