Order a replacement. I'd get two and stash the spare. The newest the original can be is 1945, and at that point a lot of Japanese QC was poor to non-existent, especially on small "minor" things.
It has probably just weakened over the years. One thing you can try, while waiting for a replacement, is to turn the spring over.
Mauser type magazine springs can often be installed upside down (yes, there is one) sometimes they work ok that way, sometimes not so much.
Any attempt to "repair" the spring by stretching usually fails, often leading to the spring completely failing and breaking soon after...