Any standard size servo will fit and any standard size 2S ROAR type battery pack will fit. I use Turnigy Nano-Tech LiPo batteries. You will want to dremel the little rounded corners in the battery tray so it sits all the way at the bottom. You will also want to get a graphite battery strap instead of the one that comes with the car that's meant for NiMh battery cells.
http://www.amainhobbies.com/product_...ery-Strap-Blue