Thanks, flyfishdave. Some very good information for me there. The current large hole is about 0.189", so to use the 0.125" diameter, I'd have to drill it out and insert a bushing or piece of tubing. I do like the idea of being able to easily change the hole size with inserts. The NM Rear Aperture looks good too, if it can just replace the the existing portion (that is, if it threads to the current windage adjustment screw. I can call and find out.

No problem with the co-witnessing, but I need the rear sight to fold down to clear my scope when mounted.

Again, very good ideas. I'm going to investigate these further.