I doubt that the charging/lighting system voltage has any, direct, relationship to the ignition...unless it's battery ignition. And I haven't taken a dwell reading on anything since the late 1970s. While I do want to understand the principles behind, well, just about everything...I also realize that there are practical considerations which keep things from spiraling into a migraine. Specifically, here, it's breaker points vs CDI and 12v vs 6v. There are very few choices in manufacturers so, it's really just a matter of sourcing the parts as a complete assembly/system, imho; ignition either works, or it doesn't. It's not like there's anything that can be adjusted, as with a carburetor, for example. And, with the exception of the CL70, all of the engines that will bolt into z50/CT70 mounts have magnetos...12v/6v doesn't make any difference; with these engines, the terms refer to the lighting/charging coils. It's only used as convenient, if somewhat misleading, nomenclature. The 6v engines came with breaker point ignition, the 12v versions CDI...the alternator assemblies are completely different, with nary a single part interchange between them.
That said, breaker point ignition vs CDI can be viewed as comparing spark output between household 120v current and static electricity you'd pick up in a dry household environment. They'll both produce a snappy, blue-white, spark but...the former has much more current (amperage) whereas the latter has much higher voltage. Doesn't take much wattage to produce a healthy spark, just voltage. High voltage + low current is easier to to handle with simple, small, electronics. Breaker points, OTOH, are physically suited to higher current draw, as evidenced by the hard alloys used for the actual contacts. Cutting to the chase...CDI coils are smaller than breaker point ignition coils, both primaries & secondaries. My guess is an equal, or greater, number of turns, using smaller gauge wire wound around thinner cores. The biggest difference is inductance. Impedance values are minuscule, almost useless (without sophisticated equipment and knowledge) other than verifying a dead coil. The bottom line, I've always viewed these ignitions as complete systems...primary coil, pickup, CDI module, secondary coil...I've seen too many problems resulting from mix & match. I apologize if this answer isn't what you were hoping for. It's a case of basic practicality, i.e. the best parts to use are the ones you can get which also work, over everything else.