I've noticed a few questions about 6 to 12 volt conversions recently.
Here's my experience, plus some stuff from a 1989 VW Trends article.
Let me repeat:  these are *my* experiences.  Yours may differ.  See
http://www.caer-galen.outlands.sca.org/disclaimer.html for the full
disclaimer.

- The wiring harness should not require replacement.  However, this
  would be a good time to check for breaks in the insulation, or in
  the wire where connectors attach.  Any problems along those lines
  should be addressed, particularly semi-broken wire.  Cleaning
  up any corrosion would be a good idea, too.

- The flywheel on stock 12 volt engines is larger than on stock 6 volt
  engines.  My experience has been that keeping the 6 volt flywheel
  (and corresponding `bendix' on the starter solenoid) works fine.  As
  a corollary, using the 6 volt starter works very well in a 12 volt
  system.  This is because you aren't running the starter motor for
  very long, so the higher voltage doesn't cause it much of a problem.
  It also causes the thing to spin faster, which makes for easier
  starting.

- Remember to replace everything that even vaguely resembles a light-
  emitting device (head lights, tail lights, running lights, ...).  In
  particular, don't forget the oil pressure warning light.  Thanks to
  opposite advice from someone that should know better, I toasted an
  engine by not doing this -- it promptly burned out, the engine had a
  leak, and you know how it goes from there.

- You'll need a new generator.  The VW Trends article suggests the
  Bosch GR26X.  It has the advantage of being the same size as your
  existing generator, so you can use the generator stand, pully, and
  belts that you've already got.  I went with a slightly larger one,
  that required swapping out all the other stuff, also.  I had
  significant problems finding belts that fit.

- If you have an electric choke, you'll need to get a 12 volt version.
  They're the same size on Solex carbs, so if that's what you have,
  you're in luck.  Since you have a 6 volt system, you probably have a
  Solex carb.  I can't imagine anyone having upgraded the carburetion
  without having changed the electrical system also (but then, I'm
  often accused of not having sufficient imagination).

- You've got a couple of choices for handling the 6 volt motor things
  (primarily the windshield wipers, but also the heater assist fan if
  you've got one).  Choice A is to put in a voltage-dropping resistor.
  This solution is quick and easy, but not a good one in my
  experience.  First of all, you end up throwing away quite a bit of
  electrical power in the form of heat in the resistor.  Secondly,
  using a resistor as a voltage drop only works if you know how much
  current you are going to be pulling (voltage = resistance *
  current).  So, you either have to measure the existing resistances
  and do some math, or hope that the off-the-shelf unit you get is
  close enough.  And you are still left with the painfully-slow 6 volt
  wipers.  Choice B is to replace the motors with their 12 volt
  equivalents.  This is certainly more work, because you have to
  physically remove the existing motor(s), find replacements that will
  fit (both into the mounting brackets, and into the shaft
  attachments).  I went the first route, and am planning on ripping it
  out and doing it right.

- I'm not real sure about the fuel gauge.  I don't have that
  particular vehicle any more, so I can't really say much on the
  topic.  I seem to remember it failed a few months after the
  conversion, though, so I suspect that it might need to be replaced.

- Although not really required, this would be a good opportunity to
  replace the plugs, wires, points, and condensor.  There's no
  difference between the 6 and 12 volt versions of these (i.e., there
  are no separate versions), but these components are subject to
  extreme conditions, and replacing them just eliminates one more
  potential problem.

Here's stuff directly cribbed from the VW Trends article:

- Parts list: headlights, coil, turnsignal/emergency flasher unit,
  all the other light bulbs, battery, voltage regulator.

- The 6 volt horn will last a while, but will eventually burn out.

- Depending on what year you have, swapping the entire wiper gear and
  motor assembly out may cause your wipers to stop in the opposite
  position than they used to.  There are replacement switches and such
  available to fix this.  This problem is why I suggested just
  replacing the motor above.

- Lots of places have conversion kits.  Call up and ask.

Hope this helps.

	Dworkin