6v regulator for what?
You can not get 12 from 6 volts without going to a step-up transformer. There are sites on the web on how to build them, and some motorcycle shops sell them ~$80 -$60. Looks like a 4"x 5" box with heat sinks on it.
Once again, the problem is that if you have a 24 watt bosche dynamo, you still are limited to 24watts. So converting to 12v this way will net you 2 amps of total power.
You can probably convert a 6v bosch dynamo to 12v just by adding more permanent magnets to the inside of the fly wheel. But without bench testing or imperical data on a "how-to" I wouldn't want to try it.
Then you'll have to convert the coil a 12v unit (lest you destroy the coil or the points) as well as all of the lighting horn.