Barrie — Sorry about that! Another ‘senior moment’! I saw the “Mag” in “Magdyno parts” and went onto autopilot.
I don’t know where you’ve got this from, but how then is/was the mag manufactured without this “loss” occurring? Wouldn’t this apparent loss be inevitable, given the construction of the mag?
On this basis almost every mag in existence will have suffered similarly, but I have bikes with mags (as apparently do many of thousands of other riders) and they happily light up the mixture most satisfactorily.
The magneto is magnetised when fully assembled. I would refer you to the Lucas publication “Remagnetisation of Magnetos”