If it is just spares, decide if you would bother fixing it at the track, then pack what you have. This can also be determined by how far it is from home. When I'm at the track that's 18 miles from home I will bring the car home for major repairs. When its a 10 hour tow, then more stuff gets brought. Same for the type of race. A 24 hour enduro (2000 miles from home) requires different spares than a single sprint race.

If you list is everything you want to bring, spares or not, just go through everything in you mind that you will do and write down everything from your alarm clock in the morning to your toothbrush before going to bed. It might be a big list, but a couple of pages of paper are way easier to waste than an hour looking for a Walmart to sell you fresh underwear at 10pm.

Don't forget things like your 'factory' shop manual and parts to fix problems on the trailer - the stuff you don't ever plan on needing.