Did a 12V conversion on my bird.
Walt Knuckles did a good article on this.
Your 6V starter will spin faster on 12V. Maybe too fast... just be ready to replace it if it burns out. I had to replace mine after a few months.
No need to replace the harness unless it is already shot... 6v wiring is generally twice as heavy as 12v wiring. Just be careful of cracked insulation. Most of my original plastic insulation that wasn't exposed to sunlight held up fine.
Replace head, tail, and turn lights. All light bulbs will eventually need to be replaced... either initially, or when they burn out. You will also need a new flasher.
No need to replace fuel or temp gauges or senders. I got a small
instrument voltage regulator for each gauge, which reduced the voltage and will extend the life of the gauge.
Replace the heater blower motor, or only use it in "low". Power windows will work, but super fast... and will burn out if you hold the switch down. Likewise with power seats.