look for a UPS battery, something like this:
http://www.amazon.com/APC-RBC2-Compa.../dp/B004H2YVPW
There is a really good explanation about batteries/amp hours/wattage on this forum, go read those, will explain what to get.
I think they weigh about 5 lbs. not enough to worry about.
I bought a Havoc for my grandson, have never seen a reverse for it... Search this forum for "reverse" you will see a lot of discussion for a GY6 reverse, but not for these. Your leg is probably the cheapest/best reverse.