I look at in terms of what will wear outor get lost and how easy is it to make.
A bolt can crack and is not a candidate for fabrication, so a spare bolt would be good. Firing pins and extractors DO wear out, so a few of those are good. An extractor plunger is a great part for getting lost, so several of those. Springs are springs, reasonable substitutes will always be around but a plunger spring, slide spring and ejector spring will be useful. Ejectors are probably never going to wear out but are handy if you are getting brass in the head, so maybe a couple of these in different types. Probably the biggest wear item on the carbine is the magazine. You can never have too many of those.