I rarely use parts from unbuilt kits for a different project. I have been known to build something to get the leftovers for a future project...
But I save all the leftovers as I go, and over the last four decades, that has created a bunch of parts I often can't find when I want them.
So I just scratch-build instead of looking. Seems more productive, and once I make what I need, I find what I was looking for without trying.
The same technique works for finding dropped/lost parts...