I don't own one (do have the big brother tho, T1 FK04
) but just looking at pictures I would think most of the 3.7V packs that pan cars run would fit. It might require some creative mounting solutions...shimming the battery strap or removing it completely and using tape, possibly adding some stops to keep the pack from sliding around if it doesn't have humps on the bottom or if the humps don't line up with the slots on the chassis, stuff like that. The GTB/13.5 would be a great match for just playing on the track, IF the GTB will fit. It has a large footprint and I don't know how much real estate is on the chassis without having one in front of me. You also may have to do some minor dremeling on the chassis cutout under the motor to get the brushless motor in there. The bulkheads and the cutout look identical to my T1FK, and I had to remove some material from the cutout to allow the motor to swing down far enough to allow for brushless-friendly gearing.
-rocky b