J.C. Whitney used to sell a kit to do just that. It would start a 6-volt system on 12 volts and a 12-volt on 24. After converting one of my tractors to 12 volt and going through several starters, I found that the original 1/0 cables were so badly corroded inside the insulation that very little voltage was getting to the starter motor. With new cables on there and a 12-volt battery, it cranks faster than it idles.
The short version of that is: try new cables first. You might be surprised.
