I think stock should be 5 cell NiMh and a specific motor (in 1/18 4wd).
Mod should be open motor battery...No restrictions! With the brushed motors available today, there is ZERO advantage (on the track) with brushless! The only reason most of us run brushless is the lack of maintainance.
This reasoning applies to most any roadcourse we would be inclined to run 1/18 on