[Phlogiston]

You can do which ever is easier. When big parts are missing I make new ones. On a lot of trim, I patch it.

If you have the patience, you can whittle down a piece to nearly the exact size. I usually try to match the broken surface, sometimes smoothing it (the broken surface) a bit, glue in the piece and take off the extra after the glue has dried. A Dremmel tool will help a lot with this.