The very first longrifle I built utilized a T/C Seneca lock, and the patent breech to match. At the time I thought it was great, but obviously, now I know what an abomination it was. Some time ago, the owner had me convert it to flint, and I replaced the trigger guard while I was at it too.
I used a Davis Late Ketland flintlock. I planed off the existing lock panel down to the bottom of the moldings, and glued on a piece of maple. I inlet the new plate, and replaced the patent breech with one I made to match the flint lock action. After restaining and refinishing the entire rifle, one cannot see the glue line, and it looks so much better than it did.