I've often idly wondered how hard it would be to mod a simplex primary A to take an alternator . . . more than a couple of people must have done it I suppose?
The outer chaincase question is one thing that could be tricky, but presumably the mounts for the stator wouldn't be so hard to figure (nor the need for a cush-drive clutch, as they can be had easily enough). But what about the crank end?
One of the things that AMC did (I have a long-term affair with AMC twins . . . yup, I know, but I shan't apologise 'cos they're excellent too . . .) was make a factory extension that converted dynamo cranks (for non s/abs later models with rubber clutches) to alternator, using a neatly made screw-in shaft. A mod that is still available. But their stators were held in the outer primary, like many another machine - which is basically pretty horrible, even with good dowelling on the chaincase.
The Beesa option, if the pix from Sluggo are typical, shows the stator on studs into the drive side case, which is much more elegant even if one overlooks the apparent stack of nuts packing things out. Stator on the crankcase and a chaincase outer with a detachable panel to allow direct measurement/tweaking of the clearance between rotor and stator . . . would be nice.
Not about to try to do it, as I'm not unhappy with the dynamo I have got, but it keeps entering my head that a neat conversion package is waiting to be put together. Mag and alternator was often regarded as a 'plus' back in the day, and made, eg, some of the Ton Ten Triumphs a more attractive proposition for some people than the dynamo options. It's certainly a decent and dead reliable mix on my P11 and 650SS Norton . . . more non-apologies . . .