I have not exactly replaced springs in windup sets, but I have taken apart a number of Edmonds-Metzel (Early American Flyer) engines and worked on them. I got a junker for $20 and took it apart to see how difficult it was to take apart. Then after acquiring other Edmonds-Metzel engines with issues, I started swapping good parts from my junker. So far, I have swapped out a broken brake lever in one engine, a bad gear in another, and a bad axle/set of geared drive wheels in another. So that $20 junker has paid off over the years.
NWL