12 volt parts are more abundant and the major components, alternator, starter, battery, bulbs are cheaper (usually). Wire gauge is smaller throughout the entire system. It would be wee to re-wire completely in such a conversion.
6 volt systems, when properly maintained (clean ground and hot connections, correct size wire and cables) are just as reliable as 12 volt. One caveat. The generator won't charge until you reach 750-800 RPM.I don't recommend this for a steady idle. Parts can be expensive. Best to locate a starter and generator shop that still does six volt. If your community has a thriving antique car club, this shouldn't be all that difficult. I have both in a couple of 2A's No problems with either.