A second plug heater has helped me in the past, especially when using the NiStarter type ( the older they are, the quicker they lose charge). If using a glodriver, a NiStarter is a good backup too.

I also carry the 1/4" wrench, a prop rod and plugs, but since using APC props, I haven't seen the need to carry a spare since they don't have the problem of breaking if you snag a tip on your shirt carrying your plane to the line like wood ones used to!

A 4-40 allen wrench might be handy should you find a loose head screw or muffler bolt, but if this happens you didn't preflight too well in the pits!

Race hard,

Drew