If you're keen on tearing down and measuring the spring dimensions to figure out the rating.
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www.planetspring.com
From there you can source a spring similar in dimensions with a smaller wire od, and check that calculator again to make sure its not TOO weak. Ideally only a 1-2 less lb/in spring.
There are other ways to skin the cat but the above allows you to return to the stock spring if you wish.
-Matt