The MJA is one of the biggest and baddest of the American Bosch MJ series. Very robust and reliable.
Here are some images from a factory owner's brochure to share to keep yours alive and well. There is some helpful factory service manual info presented as well.
Your magneto is probably out of warranty.
The hex headed screw that fastens the point spring has 10-32 threads and a Hex head that is 5MM across the flats FYI. If there is still
meat left on the point contact surfaces after dressing and polishing, by all means use them. When installing new points, make sure that
the contact surfaces line up with each other for maximum surface contact. A fine diamond lap like an Easy Lap is great for getting the jagged
edges off the points, then dress to a mirror finish with something very fine like a stone or Cratex stick. Points that are smooth will stay clean much
longer than rough. We usually aim for .015" of point clearance. If the point contacts are far out of alignment, sometimes you can slide the
phenolic pivot bushing up or down in the point lever to correct. If you find a barn or field fresh unit, always best to pull it apart, wash everything out
and repack the bearings. For the points cam, put a dab of grease the size of a match head on. An electric motor grease or a good molybdenum
product that will stay put is desired. oil and grease on the contact surfaces is a big nono.
Here is some info on setting and checking edgegap. Optimal performance and best spark is where breaking the electrical circuit occurs at an ideal interval after breaking the magnetic circuit. This interval is edgegap. If you remove the breaker plate you can see that there is a fair range of rotational positions it can be secured in. If it is an adjustable timing model, place the timinmg lever in full advance when setting the edge gap. If the edgegap is not close to right, we will have a wimpy mag. Check the edgegap after the points are properly set.
A drill bit works well for this task.