It's fairly easy. switches and wiring is all ok to leave. you will obviously need to change bulbs, dynamo, regulator, battery, indicatior relay, wipers, horn and starter, but for the starter you will possibly need to change your flywheel as well. However you may be lucky as a 67 may be a wierd 6v started with 12 teeth. (not sure on this)
https://www.karmannkonnection.com Just a quick one. I still run 6 Volt and have never had any issues, you can get 6V halogen bulbs that are bright, and as long as everything is clean and well earthed, there is no reason why you need to spend a lot of money, possible getting over any issues you have. Only one small issue is no sterio.
Good luck
You can never have too many accessories on your car. 🙂