I've had no problems over several years with an original Miller D6 dynamo and V-Reg2 6V regulator. I run an LED headlamp bulb and 10W tail lamp, for a draw of 20W with headlamp always on - well within the limits of the dynamo.
I'd be interested to hear why people conclude the original dynamos are unreliable. The only wear parts are the bearings and brushes, reasonably long-lasting and easily replaced. Armatures that are 70 years old may fail, but new ones are available for a fraction of the Alton investment.
Wanting a quadraphonic stereo or heated vest might be good reasons for a higher-output alternator setup, but lighting can be provided these days with LEDs at low current.