I have never replaced balsa core in a fiberglass hull before. I was a little worried. I found instructions on a forum board and talked to a few guys who had done it. Here is a link to instructions and my thread on the checkmate - boats forum : http://checkmate-boats.com/forums/showthread.php?t=3343

The larger area marked with a sharpie is the area to be cut out. You can see the center hole I made in order to determine the condition of the balsa core.