RE: Sea bird rebuild
I don't know what the seabird looks like however, I added a bit of lenth to one of my first planes by cutting out the material between the top and bottom spars on the wing tip and putting in a piece of ply, like a wing joiner to connect another section of wing that I built with the same airfoil as the original. The wing tips do not carry much of the load, so super strength may not be need. Another way that just came to me may be to just hotwire out a lenth of wing out of foam, put a balsa spar top and bottom and glue it flush to the squared off end of your wing and then cover it. Six inches each side would give you one foot extra wing length.