I suggest you leave it 6 volt.
Have converted cars over in the past unil I finally got wise. It's a lot of work for not much gain. If there is something wrong with your starter, ignition etc. changing to 12 volts won't fix it. You will go to a lot of work changing it, then have to fix what was broken in the first place.
Easiest and cheapest answer, fix the 6 volt.
Next easiest and cheapest, sell it and buy a 1955 up car that is already 12 volt.
If you tell me what is wrong with the car I can help you fix it a lot easier and cheaper than changing it to 12 volt.