Also Sprach CCred68046 :

A simple answer to a long problem: Buy a solid state rectifier/regulator
intended for an old motorcycle. I bought one made by Tympanium and sold
as a replacement for Triumph motorcycles. Just be careful with polarity,
the old Triumph's had a positive ground electrical system. Make sure that
you connect the negative output to ground, even though the instuctions say
positive.

I bought mine locally, there's a great bike shop near me, but here's an
online retailer: http://www.oldbritts.com/17_01233.html

Dan

--
Give a hungry man a fish and you have fed him for a day, but give him a case
of dynamite and soon the entire village will be showered with mud and water
and hard-to-identify little chunks of fish.