If you have access to a lathe, plus time and patience, you can turn the stock flywheel down to a small prophub. Then you can simply drill a hole in the appropriate place and install a magnet. I have done that for engines that I couldn't find existing prop hubs for. To me, it's a method of last resort.

If a prop hub is commercially available, get it!!! Cutting down the flywheel is just not worth the time and effort, unless you enjoy the work.