You can make new pieces and glue the new to the old. You will then need to bond the new into the old with some bracing. you will need to make some plywood strips the width of the leading and trailing edge and blend it together. After I looked at your picture again, I am thinking that I would cut off the rudder, whittle out the remainder of the damaged stab and build a new one. That might be an easier fix and I know it would be a stronger one. Good Luck, Dave