Following the lead of Brian Rupnow and others, I decided to make an "ignition box" to hold a battery and automotive coil, with leads that can be hooked up run to an engine that has points and condensor ignition. The pictures below show what I came up with - a 3d printed box and lid. As you can see, I need to do some tuning on my 3d printer to reduce the "ringing"! And I need to put some white paint in so that the letters are more visible / readable. But it works!
One side of the box has three connectors, one each for ground, points, and spark. The other side of the box has two connectors, ground and 12v, and a SPDT switch with center off. When the switch is moved to the "CHG" position, the battery is connected directly to the GND and 12V connectors, allowing it to be charged with an external charger. When it is moved to the "RUN" position, it is wired up to run the engine. And in the middle, of course, nothing is connected, allowing a way to switch off the engine.
It is not obvious from these pictures, but the box is printed so that there are recesses in the box itself and in the lid that "capture" the battery and the coil and hold them in place. You can see a bit of that in the next post, where I have a picture of the box being printed.