Two totally different guns. Elcee's didn't come without sideplates. They were/are sidelocks. The Hunter Fulton is a boxlock.
Unless a family heirloom your quest is going to be financially an unsound venture. The cost of new components will exceed the value of your gun when done.
Chances of finding components is slim. If you succeed, then you must remember that each gun was hand fitted. After that comes the checkering and final finish. Not a job for the faint of heart!