I'm not sure if your Cub has a magneto, or a Battery Ignition unit.
There is a page in the Cub service manual about timing the engine with a timing light.
I finally got a timing light and now I am looking for this page in the service manual. I watched a youtube video of someone doing it on a car but I am looking for more info on the tractor before I venture forth.
Did you recall the section its under? I did a quick search and I didn't see much on the forum about how to use at timing light.