I use a Magneto Guys timing light, very useful and accurate. Worth every penny, although the insulation on the alligator clips makes them a bit fiddly to get attached properly

I have a mate who is a light aircraft mechanic so borrow his when I need too.. Handy tool, but if you use the old fag paper method, bin the fag paper and use a slice of crisp packet, you can put tension on these without it tearing.....

If you look in Irving’s Tuning for Speed there’s an illustration of a torch bulb and battery being used to set mag timing. All very mid 20th century!