If you really want some interesting reading on sidelock/ sidelock plate and parts inletting get a copy of "The Gumsmith of Grenville County" (Building the American Longrifle) by Peter A. Alexander. If you are good with Acraglass Gel and spray release agent you can build-up the internal missing parts with that product (Brownells)
Believe me when I tell you it is not as easy as just removing all the wood that does not resemble a shotgun stock. It takes a lot of careful spotting-in and paring away as well as a minds eye for what you want the result to be.
Good luck!!
