Al, you can see from your good pick up that the carbon brush is spring loaded, so you need to account for the spring and the brush. The spring could still be in the pickup, or in the magneto. So first thing is a good look, torch/mirror trick, next a piece of rag or paper towel worked in and persuaded round the slip ring....this should find the remains of the missing parts with a bit of luck.
If you have no success, but are sure the bits are in there, then the end housing needs to come off, for a good look inside. With no primary case on the engine this can be done, but the usual way is to remove the magneto from the engine, which will loose the set timing when the drive gear or auto unit comes off the tapered armature shaft, and then dismantle on the bench.
There is plenty of information about stripping and checking the magneto, and ways of releasing the drive gear from the armature. Plus a few thing to watch, all in the literature section of the Forum. Also Priory Magnetos and Brightspark Magnetos are highly recommended, with very useful of information on their respective websites.
Try the easy stuff first, fingers crossed!
For a weaker mixture the needle needs to be lowered, so use the top groove for the clip.
Swarfy.