Alternators charge at lower r.p.m and are much cheaper than the cost of having a generator repaired.
I can buy a Delco S-10 alternator for about $40. It's internally regulated and even if I have to buy a new 12v battery and new lamps, it's cheaper to convert than to repair/replace a generator or regulator.
A good 6v system will start fine and you don't need anything better than that but 12v systems are more fault tollerant when it comes to slightly corroded connections. With 12v you don't need as large of conductor to deliver the same power. Power (Watts) = Volts x Amps. If you double the volts you deliver the same power with half the amps so the conductor can be smaller.
Since a 6v starter will handle 12v without any problem and polarity doesn't matter to a starter, most who convert to 12v are very happy with the results.
I converted my MF2135 when my generator quit and have had no problem since.
Almost forgot, if you have a distributor ignition, you'll want to add a resistor to your ignition circuit to drop the current going to your points and coil, or switch to a 12v coil. Magnetos don't care because they don't draw from the battery system.