Some came with Miller, and later ones came with Lucas. Not sure when they changed supplier, but my '64 Clubman came with all Miller electrical equipment.
The E3L Dynamo was the longer Lucas one, and put out about 60 watt, and the E3H was shorter and put out 40watt. (I think the Miller was 40watt too).
Best bet is the E3L, and yes they can be rewound to give 12v or you can use something like the Criterion 6 to 12volt solid-state converter.
I can't remember whether it has a taper ended shaft or a keyway for the drivebelt pulley, sorry.