Great question Justin! I have shot this rifle a fair amount as has Daryl, and we have never had issues with the frizzen spring. When it broke on the first shot, I didn't notice anything wrong until I found the upper leaf laying on the table. The temperature was just below zero C, so it wasn't the cold tht was the issue. I made the spring from a TOW casting when I made the lock those years ago, and obviously, had the temper a little too hard.
I'm pretty sure a large Siler frizzen spring will replace it, but it won't have the shape and filing of the original Verner spring.