I bought an older 25 hp Yamaha, elec. start motor. Not wanting to carry a big 50# battery around, I tried a battery from my garden tractor ($19.95 and very light.
When it worked, I bought a new one. It's worked fine.
My son did that also with a motor cycle battery! The thing you will find is that it won't last as long, because of the draw down strain, over time! His lasted about 7 months!, even with taking out and charging on a quality house charger!