General Battery Info:
The Yuasa part # for the stock battery is YB10L-B2.
Equivalent Battery @ Walmart is: ES12BS (courtesy Srinath)
Links:
Pablo's page on Battery and How to test your battery (http://pantablo500.tripod.com/pablosgs500page/id21.html)
Pablo's page on Walmart Battery (http://pantablo500.tripod.com/pablosgs500page/id41.html)