Reminds me of my Kyosho HOR. The rider moves when steering the bike. I used a 800mah 2s lipo on my HOR and the battery was small enough to fit in the battery compartment. I also added some stick on lead weights (great planes) to help keep the bike planted. This might help your build.
Here is my old build thread there might be some useful info in it since these are similar builds
http://www.rcuniverse.com/forum/rc-m...nda-build.html