How to fix a spring in brake lever?
This sort of component-level repair is rare in cycling.  Generally speaking a shop would fit a new replacement brake lever and clamp.  Moving sub-components around would be the realm of someone valuing a period-correct part over functionality.

I'd replace both brake levers with either new or a good quality used replacement.

Why both?  So they feel the same when in use, with similar action and pivot distances.

Brakes are a critical safety component, and I'd think very carefully before twiddling internal parts around and using a generic item as a replacement.