The HT ignition coil has two terminals, the black wire runs from the magneto to the coil, and then to the kill switch, the other terminal on the kill switch goes to ground. This side is only grounded when the switch is in the off position (will not spark). The other terminal of the coil is grounded to the chassis. This means that you can run the black wire from the magneto directly to the ht coil terminal and ground the other terminal for testing. Alternatively you can disconnect the black wire from the magneto, and check the wire on the harness that it connects to for continuity to the chassis. You should have no continuity with the switch in the ON position, and continuity with the switch in the off position.
Simplified, for spark you simply need to run the black wire to one side of the ht ignition coil, ground the other side of the coil, ground the blue magneto wire, and the yellow wire shouldn't matter, it can be isolated.