Yeah, a magneto like that will knock flat. What's even worse is the ones for a distillate engine. That stuff is basically kerosene, and most tractors that burn it have dual plugs and massive magnetos. Touch a wire on one of those, and it'll mess you up pretty bad. Touch it with your other hand on the block, and chances are you won't be getting up.
My uncle had an old D9 with a 2cyl gas engine hanging off the side of it. It's a lot of load on it to get that old diesel spinning over.