Lawnmower batteries are intended to be used at high amperage rates for very short times, not discharged and recharged constantly. The SLA batteries you get from the internet or your local supplier are made for this type of application, are just as cheap, much safer, and should give much better life/performance than lawnmower batteries.
another trend is to use Lithium (LI-insert po/fo/mg/iron and so on) batteries to power your scooter. this is a very good choice, but you must do your research. they are half the weight, have 3x longer life, give greater power to your scooter and also cost 3x as much up front. they also can catch FIRE. so watch out. understand what your getting into, know everything about your new battery setup and learn how to charge them, that is when they are most likely to catch fire.
FYI... we have had a user's new lipo battery set catch fire: http://www.modifiedelectricscooters.com ... IC_ID=1496
Speed, Distance, Torque... Pick any 2