I carry a spare matching speed controller in the event of ESC failure, a secondary set of batteries for swapping out in between fights and a spare set of wheels in case the drive wheels are irreparably damaged. Apart from these items, little things like spare wire, LEDs and a good set of spare set of screws/nuts and bolts.

Those, coupled with a toolbox of a soldering iron, some screwdrivers, the battery charger + bag and my Dremel has seen me though several events.