The pan seat was an option. I have a C with one and it is original. If you go back to less than a 12v battery, consider going to an 8v. You can modify your 6v generator to charge at 8v and it will start stronger and you don't have to change any light bulbs.
Personally, if you're going to use the tractor and not show it, I'd convert to 12v negative ground. A new internally regulated alternator will cost you less than the first repair on either your existing generator or external regulator. There's not much wiring on a C to start with, so removing everything and starting from scratch wouldn't even be too large a task. If you have a 6v starter, you don't have to replace it but don't lay on the starter and let it run for long at 12v. You shouldn't have to anyway.
Look over the Article Archives for 6v to 12v conversion and Upgrading Your Charging System.
If you have any questions or need some particular help, I have the C operator's manual and could help you with the wiring details.