I always like to have spares of anything relatively inexpensive such as electrical switches (toggle & push button), fuel pumps, and starter solenoid. Things that can be swapped out easily when trying to troubleshoot gremlins.
I also started to keep pieces of aluminum and rivets to hold together fiberglass pieces temporarily. Just one step above duct tape bodywork.
A not so cheap part that is good to have is a spare muffler. Unless your smart enough to tether it the pipes, Guys seem to overlook a failing tab and tend to loose them at the track. always easier to slip another back on than have to find it, re-shape it, and re-install it in a rush. I like to fix things during the week, not at the races.