A blackpowder gunsmith can make a spring for you.  A few regular gunsmiths, too.  You could make one if you're willing to learn and collect up a couple of tools.  Don't toss the broken spring, it's needed for a pattern.  If you are close to Trinidad, go to the gunsmithing school to ask for help.

Does the hole through the spring serve a purpose?  If it's required, a new spring should be as wide as possible and the hole as small as possible so it will fit, and thicker at least at the hole to reduce the stress at the cross section.

Increasing the thickness is most effective, the stress will decrease with the thickness squared.  Let's say the thickness is increased from 0.040 to 0.050 inches at the hole.  The new stress is
(0.040/0.050)^2 =  0.64 times the original stress.  A rule of thumb for improving fatigue life is, the life will double for every 10% reduction in stress.

The upshot is, not much change is needed to make a big improvement.  But get rid of the hole if possible.