Your existing coil box appears similar if not identical to units used on Model T Fords. If so, the engine side would be laid out like this Kingston unit. the Ford diagram would apply. Note that your engine would have a timer wired to the coil box, a typical timer is pictured. The wiring diagram shows 1911 Hudson with a special coilbox that controls both that box and a magneto. The idea is to start on coils using a battery then switch over and run on the magneto. Your existing magneto with start the engine if you crank fast enough, it is not capable of making spark from battery power. The idea in the old days was to prime the engine, then turn on a battery powered system that would spark the plugs thereby starting the engine with not hand cranking needed. I am going to guess that your engine has 2 sets of spark plugs. What does the engine side of the coilbox look like?