You can count the teeth on the oil pump shaft gear and the gear on the camshaft that drives it.
12 on oil pump shaft and 24 on camshaft are set up for magneto.
16 teeth on each are set up for distribuitor.
I have been told that if you just put a distribuitor on a magneto drive, it may start but will do all kinds of funny pops. This from the extra sparks when the valves are open, I guess.