While this might be true, it's important to note that total power is a function of both current and voltage. The total power available at 12V is not as high as it is at 24V. In addition, the wiring within the motor limits the amount of current that motor can handle. Exceed the capacity of the motor windings and you'll burn it out. Increasing the voltage reduces the current requirements as well as increasing the total power.
This is interesting. Consider the following:
2, 12v batteries capable of delivering 10A each.
In series you have 24V and 10A. This gives 240W of power.
In parallel you have 12V and 20A. This gives 240W of power.
Looks like the power is the same - all things being equal and the current being unlimited.
Torque however is a result of RPM coupled with HP.
I can tell you that running a motor designed to run at 12V WILL burn out faster at 24V than at 12V. It WILL spin faster at a cost - there is no such thing as a free lunch. (But I'll probably end up doing it to start my gas engine.)
The windings and field magnets are chosen to handle a certain amount of current at a narrow voltage range - simple design parameters. The windings do not limit the current. They are simply bare conductors with a varnish coating. Too much current and the wires either burn through creating an "open" or they short together. Either way = dead motor.
Too much voltage (even if the current is limited) will produce too much electromagnetic field resulting in an undue amount of heat build up. The motor cooks.
At short bursts there
shouldn't be a problem with running a starter at 24V. Whatever works for you.
As mentioned above by Rodney, use the appropriate size wire and connections. It DOES make a difference.
There are a variety of starter motors for ATVs, motorcycles, riding lawn mowers, etc. that may provide the torque at the voltage you are looking for. It's been done before so I thought I'd share that little tid-bit. You might be surprised at what you find along side the road, in a junkyard, repair shop, etc. DIYs love doing this kind of stuff!
I use to use two 1500mAh, 7.2V, sub-C packs on my Tower starter. It would start up to 1.08 2-strokes and 1.20 4-strokes. Compression had a lot to do with whether or not the engine would turn over.
Best of luck and Happy New Year!
Jeff