Definitely get some spare spur gears- these to me are a must have spares item.
Its also handy to have spare pinions as well....I generally pre-mount these on a spare clutch bell to facilitate a quicker change-over if need be on race day.
One spare I`m glad i had was an spare rear diff housing....a stone on race day mashed my rear diff teeth rather badly, so i was glad to have a 3-Racing aluminium as spare
...hopefully the teeth on this will be a little more rock "resistant".
Also get a spare pulley set.