Hi,
It's important that the ignition timing be set right, so the engine has as much power as it should. Especially when you are using a rotary mower, and need power to run it.
You can time the magneto like the Cub owner's manual says, or use a timing light when the engine is running. When the engine is running at slow idle speed, I think the mark on the front pulley should be about 3/8" - 1/2" before the pointer.
The magneto doesn't have any built in spark advance. When you start the engine, and the magneto is making snapping noises, the timing is being retarded, so it is firing at TDC.
Then when the engine starts, and the snapping stops, when the engine speeds up, the timing advances some. The manual says it is 13 degrees before TDC.
The timing would always be at 13 degrees before TDC when the snapping is not happening.
Below is a page from the 1950 Cub owner's manual telling how to time the magneto.
The timing light is easier, since the magneto is on the engine.
https://farmallcub.club/rudi/farmallcub.com/rudi_cub/www.cleancomputes.com/Cub/Cub Owner's Manuals/McCormick Farmall Cub Owner's Manual 1950/Page 32.jpg
If your Cub is 6 volts, and the timing light is 12 volts, you can connect it to a 12 volt battery, it's probably brighter using 12 volts.