Retarding the Stock Ignition Spark

Until you can afford to get a good aftermarket CDI you can retard the whole timing curve 5 degrees which will give it more top RPM power. All you have to do is dremel off a section of the woodruff key so it is like this drawing shows:

Since the magneto turns clockwise by making the magneto more CCW it causes the spark to happen later (retarded which is better for top end power). On motorcycles the shaft turns CCW. This engine is opposite because it doesn't have the extra shaft that motorcycles have for the gears.

Doing this will leave 1mm play there but if the fit between magneto and shaft is good then just the tightness from the nut keeps it in place.
Check the fit this way: Remove the magneto and the woodruff and put the magneto back on the shaft and then rotate it without rotating the shaft.
Either on the magneto or the shaft you will see telltale marks where the high spots met. If that happens then they aren't mating well and you need to use valve grinding paste. Put it on the shaft and manually rotate the magneto. That will grind down the high spots. Afterwards clean it off good.

If you want to make sure it doesn't move from the new position you can buy a woodruff key 1mm wider and grind it down accordingly.
But run the modified stock one for a while and then take it apart and see if it has moved. If it has then maybe you should buy the thicker woodruff key.

testimony: “I dremeled the little woodruff key. Man it feels like the faster I go the harder it pulls. I love it! Starts much easier too. Right when I engage the clutch it fires right up. That was the best thing i have done so far. It runs so much better. I even have more torque from a slow roll not even under WOT which I wasn’t expecting.”

Increasing Spark Strength

This is necessary if you have high engine compression because the higher the compression, the higher the spark voltage needs to be in order for the spark to happen.
This is also necessary if your squish clearance is minimal and there is a high squish velocity.
To increase spark strength you need a higher voltage output from the stator coil and you can have that with a fatter stator coil and a stronger magnet. But don't use these with a standard china girl CDI because it already produces a too advanced ignition timing. A higher stator coil voltage will make it even more advanced which is really hard on the conrod bearings. You'll need the Jaguar CDI or a similar one that produces a true 2 stroke timing curve.