You might consider putting two of those batteries in parallel if you can figure out some place to mount the second one. That will give you twice as much cranking amperage with the same 6 volts. Be sure the generator is producing 7.5 volts against a fully-charged battery after 20-30 minutes of running time at fast idle after starting. 6V batteries take a long time to recharge after starting, especially if you have to crank the engine for more than a few seconds.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!