Have you tried fitting your new frizzen spring to one of the deluxe Silers or the Chamber's Golden Age? Those already have an internal screw, and I imagine that it would fit right in, unless the distance from the from the pin to the screw is different (i haven't measured but I would bet a considerable sum that they are the same). What is the throw of the cock?
As a more general observation to you, Jim and Barbie, and whoever else might be interested: A set of aftermarket parts for the Siler that are specifically made to be easily customized - a frizzen like this but with the finale left blank and oversized, a cock left a bit big below the neck and with an oversized crest (that bit behind the jaws, I don't know what else to call it), perhaps a pan with the bridle arm left a bit large and unshaped - would be really nice to have around and would complement the Gunmaker's locks very nicely. I don't know it would be economically feasible, but Track of the Wolf is no longer carrying the reconversion castings they used to sell, and I know folks used to use those to customize new locks, so perhaps there is a niche there.