Don't assume the rest of the switch is OK.
The headlight power does go through this switch, turning it off (presumably to reduce power draw) when starting. The internal parts on original switches have generally disintegrated, and the likely reason for the missing button is that a PO opened the switch pod to see why the switch wasn't working... whereupon parts whizzed around the room. Bypassing the headlight defeat is easy, and the bike runs fine without electric start. There are many SOHC4 bikes running like that!