The easiest way to English your lock, is to file the pan round. You can also file to hammer to give it a pigeon breasted look, but that's optional.
I'm building an A.Verner pistol, and from photos of originals, he used locks that had similar shapes to your Siler, but had rounded pans. So I used a Stan Hollenbuagh lock and did some filing...turned out very nice. a year or so ago, someone on this site did the same to a couple of Siler locks, and they too look great. If you inlet a new lock, you're going to run into lots of problems.