Motorizing a DC generator

I have a gasoline engine driven welder which has a 40v dc generator with a 200 amp max welding output.  The gas engine is a four cylinder of 162 cu in displacement and 6/1 compression ratio.  The engine is hand crank started with no provision for a starter.  I start the engine by connecting two 12v "standard" car batteries wired in series (24v) to the welding leads with the welder output controls turned down to minimum.  The generator is thereby motorized and the engine starts.  I tried a single 12v battery which didn't work; I didn't try three 12v batteries because I didn't need to.  Now, I'd like to optimize the process.

What is the effect of using 36v vs 24v?  That is, could I use three smaller 12v batteries (e.g., motorcycle batteries) instead of the two larger 12v car batteries?  Also, if less than the 40 volts of the generator (i.e., 24v or 36v) will work, would it hurt to use more volts, say 48v, than the generator's output? 

Interesting, sounds like you will sort it out without any help. If you have access to a few meters, you should be able to make some measurements of the cranking voltage and current, and work out from that what the minimum requirements are.
You may not get a basic meter with a good enough current range, if so, use a very low value series resistor in one lead and a voltmeter to get your current reading.
Incidentally, petrol powered utility vehicles made by Coulmbia Par cars use the same principle. You plant your foot on the accelerator pedal, which cranks the motor from the 12V generator until it starts, when running it subsequently charges the battery.
Cheers,
Richard