The lock can be tuned, and you probably can obtain a replacement frizzen spring. I am assuming that is the spring you mean is too strong. I suspect that the pivot screw has not been oiled, the cam and the spot on the upper spring arm has not been lubed, and the screw that holds the frizzen in the bridle may be cranked down too hard. All those things can cause a stiff frizzen. Burrs, on parts that rub where they shouldn't are another source of this problem.
Tuning a lock involves smoothing all the adjoining parts, particular those that move next to each other. There should be no rub marks on the lock plate from any moving part. Where parts meet, there should be clean surfaces, that are polished smooth, and then oiled frequently. You might be surprised at how much crud slows the opening of the frizzen, and how easy it is to rust that pivot screw/pin when you forget to oil it after cleaning the lock.
See my article on shooting and Tuning a flintlock under " articles, " at the Member Resources section at the top of the forum. I think its right below my article on controlling heavy recoil. I have helped several members of this forum tune their locks. Contact me by PM if that is what you would like to try, before buying a replacement lock. Check with L&R Locks for a replacement. They may have one.