Aye Leonard,
So, out in the workshop today, with the stove going and the temperature hovering at -4°C inside.

Must be daft!
I have had a look at the rotor through a watchmakers loup at x5 magnification and the only thing that I came up with was a flake of steel, about 3mm long x 1mm wide, attached to the end of the more central trigger point inside the stator. I have de-greased the whole stator, blown it all out with air at 80psi and then wiped everything with a lint free cloth and inspected for more small slivers of steel attached to the composite magnets/timing trigger points until I couldn't see anything when using the x5 loup.
With other bikes partialy dismantled for an annual checkover, space is a bit tight to fire up The Minx and I didn't want to open the doors wide for ventilation, with -6°C ambient temperature. So, I have yet to see if the removal of a small sliver of steel from the end of one of the timiming trigger points has affected a cure. I will post after testing.
My rotor part number is: 31.96.91 and I too have the rectifier/regulator that is an alloy box with a detachable alloy lid, revealing all of the components. It is not sealed and has no affect on our problem, as the ignition is a separate circuit from the charging circuit. As I have previously explained, my OME trigger coil appeared to be OK when tested for resistance and signal voltage, but only produced the positive half of a sine wave signal and not the corresponding negative and so it was replaced with an Old New Stock Ducati Electronica trigger coil, purported to be of the "correct" type! However, if the sliver of steel was in place on the end of the timing trigger point at the time of testing and this is the reason for the lack of ignition advance, there may be nothing wrong with my factory fitted trigger coil.

I may yet have to disassemble the OME trigger coil from the stator and replace it with the factory fitted coil, in case the OME is of the incorrect type to match the CDI/HT module.
In the meantime, I have attached 4 images showing that my trigger points are in good condition, the timing marks and alignments, with the keyway on the rotor, the 3mm sliver of steel attached to the trigger point, for information and reference.
As for your query
There is of course the question of magnetism has this reduced over the years and as they are composite magnets I am not sure if they can be re - magnetised.
Paul Compton (EVguru) offers a re-magnetising service for Moto Morini rotors to members of the MRC. Personally, I do not think that my rotor has become de-magnetised, but you may feel yours requires attention and I would advise that you contact Paul directly to enquire about his service. You will have seen that he is aware of our cantankerous 125H's and he too, at one time, owned a 125H, but his didn't develop this "il folletto"!
Good health, Bill