Some people "upgrade" to two 6 volt golf cart batteries, hooked up in series to make a 12 volt battery.  They tend to be very heavy-duty so they should last a long time if you take care of them.  Make sure you keep the water topped off, charge them at least once a month over the summer and at least once over the winter if you store it outside.  If it is inside where it is warm you need to keep charging it every month.  Check out this battery FAQ:   http://www.windsun.com/Batteries/Battery_FAQ.htm - http://www.windsun.com/Batteries/Battery_FAQ.htm

When you do pull the cover off look for the amp hour rating on one of the batteries.  Since they are in series - increasing the voltage - the amp rating of just one battery is what your battery has available (when you parallel batteries you increase the amps while keeping the voltage the same, exactly opposite of your setup).  Be sure to only use half of those amps most of the time to preserve the life of the battery, but you can occasionally go up to 80%.  Never go above 80%.