Replacing the old mainspring with a new one is not that difficult. New mainsprings come pre-loaded and wound up in a tiny plastic disc. You simply pull out the old mainspring (wear eye protection while doing this because it will suddenly uncoil and expand) and then line up the disc containing the new one with the barrel and push the new mainspring into position. Once it's in, you have to insert the rachet wheel staff and make sure it engages the hole at the inner end of the spring. Then insert the barrel gear with its new sprng and staff into the movement and reattach the rachet wheel. If you do this once, you'll be surprised at how easy it is.
As an alternative to this procedure, you could just exchange the barrel gear for one from another scrap movement and see if that helps the problem.
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