Just get an electronic control box, DVr2 is what I have used and very pleased with. Easy wiring just use the wires from existing regulator, only 4, so no electrical knowledge needed, just follow the instructions.. Put in the battery and change the bulbs. The horn will work fine with 12 volts, you may even hear it.
You can even use the regulator to work as 6 volts until you get round to getting a new battery. Only one wire difference.
Only decision to make is what earth polarity do you want, I would suggest -ve earth as most things you buy now like it this way round.
You may have to Polarise the dynamo to change the polarity, easy done, without the regulator connected, run a temporary wire from the non-earthed side of the battery (ie +ve if using -ve earth) and touch it on the F tag of the dynamo, it will flash with a spark. Remove this wire and connect the regulator. The "flashing" of the dynamo just changes the polarity of the magnetism of the field coil so it now generates the voltage in the required way.
Any problems just list on here and help will follow.
Good luck.