It looks like some time someone has mucked about with the wiring as there is no dynamo on the bike and everything is run off a 2 wire 12v alternator via a voltage regulator with bulbs upgraded to 12v.
Can you just run the later 12V A65 set up with the rectifier and Zenner? Its simple and works.
The mucker-abouter has maybe done quite a good thing already if running with a decent combined rectifier / regulator. 'Better' than a zener diode to be honest and just as simple. Probably worth identifying exactly what bits you've got, and what version of alternator stator before investing hard-earned.
If a lot of power is still going to be needed to support a radio etc, then I'd have thought a really good bet if originality isn't essential would be to fit a three-phase alternator for its low- and mid-range grunt, with a reg/rect to match.
Lots of ways to skin the AC 'lectrics cat, mostly better than original in terms of managing loads and battery, and all of them eliminating half a tin of spaghetti hoops in the headlamp nacelle etc.
However, the dynamo (dummy or just a blanking plate at least) will obvs be needed to make things look right and block the big hole!
