You'll get a fair few replies on this one I reckon Chris!
Nothing very difficult, but some decisions to make, and a few bucks to invest (or more than a few if you go the whole hog).
First off, I'd stick with your magneto unless you really want to go electronic?
For the charging side:
The simpler steps involve:
* choosing a regulator - the DVR2, N for negative version would be most people's choice. About fifty GBP + shipping I think
* repolarising the dynamo by flashing the field (see many posts here for that -No, see Colsbeeza's link!)
* reversing the wiring on the ammeter so the needle moves the right way
* 12v battery and bulbs or LEDs as you say
* some would say replace the horn - but I never have. As long as you don't sit with yer finger on the button for ages, the 6v one will be fine - and louder.
But then you have to decide whether you want to continue with your dynamo as it is, or replace the armature and field coil with a pair of '12v' finer-wound items.
It'll work as it is but will need more revs to cut in and start charging and the field coil gets more of a workout. (As Colsbeeza also just said). If you're out in the sticks and don't have to trickle around in traffic it's not such a hassle and can be mitigated in part by fitting a belt drive conversion which gears the dynamo up a bit. More cost there, not sure how much these days. LEDs help a lot in this case too.
If you want to upgrade the dynamo though, parts are easily available and the job should be do-able for somewhere in the 150-200GBP region + postage, depending on whether you'd be replacing brushes, bearings etc at the same time.
Another approach would be to fit an Alton alternator in place of the dynamo. Straight fit, and regarded (these days if not always in the past) as a good practical option. At a high price though. See
https://www.alton-france.com/generators/So, several ways to skin this cat, all of them work, much depends on what you're going to be doing with the bike really.
Personally, I've continued to run standard dynamos with DVR2 regulators set to 12v, and the combination has served me for well on the various bikes I've got. I've had mechanical failures, but never so far a field coil meltdown. Friends who live in more urban environments, or who tend to do more trickling around prefer to upgrade the windings. And, this being France where they are made, many others use Altons.
