The standard way on an OS 160 twin is to wire the plugs in series and use a 3 volt source. A hot goes to one plug, the engine is a jumper, then out of the other plug back to the source. (I guess this is standard, it's the way mine was wired. You could run separate 1.5V wires in parallel from a battery of that voltage. Probably separate 1.5V batteries would be better.)