I know this is a well worn topic, but here's my contribution: The starter on my newly acquired 53 Commander turned slowly, growling all the way. I was sure it needed a rebuild, but then noticed the battery cables were 4 gauge--pretty thin for 6 volts. Tractor Supply Co had lots of heavy duty 1 gauge cables of all lengths. Installed a set of them and bingo! She spins like a champ, as fast as any 12 volt would. There is a 12V converter already installed to run an electric fuel pump, so I don't see any good reason to even think about a complete conversion. The total cost was just a bit over $26 for two terminal cables plus the cable link from the solenoid to the starter. Nice to get off that cheap.
REPorter