Right on the head, Ian. The problem is this 70 year old guy (me) who just got out of the hospital after being hit by an SUV at highway speed on his (modern) Triumph. The knees are not working right and it seems like it's going to be a while before they get strong enough to kick over anything with force, even something with a 4.5 compression. Also, it seems like it would require less work to simply add magnets rather than take apart the 80-year old mag and remagnetize.
A magneto should always be magnetized with it fully assembled. It can be done with it apart but you will loose about 20% in magnetic remanence (field strength)once assembled. But it will still work fine, especially on a low compression side valve.
Some more information regarding the theory and practice of remagnetisation would be useful if anyone here is fully conversant with the subject..I always thought the mags were done when they were apart...Ian