The ignition timing is set by two things. The flywheel key and the air gap of the coil from the flywheel. If you are just replacing a coil and not using this engine for some sort of performance application, then you use a standard zero-offset flywheel key. If the flywheel hasn't been off, then you shouldn't have to worry with that.
For adjusting the coil's air gap, there are all sorts of methods and measurements. I prefer to use a folded up dollar bill myself. Fold the dollar bill in half and place it over the magnets on the flywheel. Spin the flywheel so that the magnets pull the coil to the flywheel, with the dollar bill in between, and tighten the retaining screws. Turn the flywheel so the dollar comes out and keep spinning it around a few times so that you can make sure it's not rubbing.
I prefer this method as it puts the coil a little closer than stock specs and is easy. By putting the coil a little closer, you advance the spark a little and the engine will perform a little better at operating speed.
If you are using this engine for a performance application such as Kart racing, then you use offset keys for the specific track or race.
Let me know if this helps or you have any trouble. Thanks, PK.
Okay, that's a little different. The easiest way is to put the coil on and turn it all the way to the left and then back to the right about halfway and tighten it down. Put two layers of blue painters tape on the flywheel over the magnets and put the flywheel on the crank. You don't need to put the nut on. Spin the flywheel around a few times and take it back off and check the tape for marks from the coil. You may have to do this a few times. What you want is the coil to put a rub mark into the top layer of tape without cutting through to the second. After you get that, remove the tape and spin the flywheel around and make sure nothing is rubbing.
These are a bit of a pain. There are other methods for doing these, but this is the easiest I've found.