Sorry to see the soft knee. It's a depressing sight but not too surprising.
Some “experts” say that the chainplate knee simple supplies
a stable form for the fiberglass tabbing and that it is the tabbing that is the
structural connection. This philosophy presupposes that the tabbing is heavy
enough, that the tabbing was designed to work without support of the having a
solid wood knee properly bonded to it, etc.
Having given proper deference to that point of view I can say
that I think you should cut/grind off the old, questionable knees and start
from scratch. Use the old knee as a pattern if you can. If I were doing it, I
would build a sandwich of a layer of ¼ or 1/3 inch McMaster fiberglass, a ½ or ¾
inch layer of marine plywood, another layer of McMaster fiberglass, another
layer of plywood and a final layer of McMaster Fiberglass. Glue the layers together
with 404 and saturate everything in lots of epoxy. Clean/sand the topsides area where the knee
will be fastened down to virgin glass, sink the knee into a thickened mush of
404, create your fillets, and glass the whole thing in 24 oz biaxial.
Just a note: when using the McMaster fiberglass, be sure to clean both sides thoroughly with acetone and rough sand both sides with 60# sandpaper.