I've never converted a bike, but I have done a couple trucks. Shouldn't be a big deal, just take the approriate parts off similar models. The way Honda operated, it wouldn't surprise me to find the guts of the generators on later model 12V systems bikes will bolt onto earlier 6Vs. Then it's just upgrading the points and coils, starter if it had one, reg and recti, and of course, all the lightbulbs. Check on the Harley sites, those guys do 6V to 12V conversions all the time.
If you were to get the bike for free, then it would be reasonable to convert, just for the ease of finding parts in the future.
Chris in Boston