HiI have only been fiddling about with chainsaws for a couple of decades and never needed to replace a recoil spring on a saw but have replaced many on motor mowers, small motors for water pumps, etc.
You'll need to find a spring with similar dimensions which should be easy, see eBay, then attend to each end appropriately to make it fit your application.
You may need to cut notches with a Dremel, drill holes with a cobalt tipped drill bit or use heat to make a bend or curl.
A bit trickier is when the pawl mechanism busts - that can be a real royal PITA to fix.
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