Bill,I did the same thing too back in the early 70's too. The alternator I used was off a Mini Cooper that had been wrecked. Mine has been on my B since then too.I went through 2 regulators before I found a poor connection between the alternator and the ampmeter.Instant death! I think the benefit of using this type of automotive gear is that it is well proven unsophisticated and readily available.I didn't have any flash tools at the time so most was done in the fitters room at work. I always carry a spare belt in the headlight so I don't get stuck.In the 70 s, I did a quick Bodge, Cut a Dynamo body in half, Fitted 2 drive ends and put a V pulley on the end.
This Jack Shaft has a V belt to run an Alternator from an old British Mini car, Built in regulator, More Power than I will ever need !, Does not over charge a 4 amp cheap battery, 12 v.
IT'S STILL ON THERE .
For my new Bike, I have a Walkernator, Ready to fit, Very well made, Much neater than my effort, Better belt type. Cheers Bill.