Mine 3.7V 400mah 1S2P battery run-time is like 3 times more than the stock nimh battery. The lipo battery are lighter, i run it at 3.7V giving me a much smoother acceleration and the high capacity allow me to run for 3 times more than the stock battery do.
I order mine lipo battery from oversea online. And build it myself(DIY). Btw i've one spare 3.7V 400mah lipo for sale, the battery(2 cells) itself cost me 18+ plus shipping. I'm selling it at 20bucks to cover the wiring, shrink tude and connector cost.