Conversion with 12V generator - by Patrick H. Dykes
This is an easy way that retains the originality you may be concerned with. Simply obtain a corresponding 12V generator (making minor bracket or mount change), or have the existing 6V generator converted by installing 12V field coils into the existing generator case.
How could it be easier?
(Along with original appearance, there is another distinct benefit to this approach, and that is, that a generator will charge a dead battery; an alternator will not! In fact, operating an alternator in a completely discharged system can damage the alternator, since it requires an electrical source to energize. It is true, of course, that any generator system will show "discharge" at idle, while an alternator does not. With the quick recovery of the generator system at "r's", however, only the most severe loads would overtax it. But, it's your decision!)