Lawrence~
Are/were the tailboards Masonite (fibreboard) or plywood ? Any hardware that might run from the keel up through the boards ?
I would saw out try to saw out the originals and then replace with 1/4-inch AC plywood. If there is no hardware, I would remove the existing on my bandsaw. If any doubt, I would try a long blade on a sawzall (with the body clamped to my bench).
I would set the new tailboards in thickened epoxy or a paintable caulk. As there is likely to be some loss of cork at the edges during the reconstructive surgery, I would add some cork dust (or other filler) to fair the body onto the upper and lower tail surfaces. And, any new boards should be thoroughly sealed - with either epoxy or spar varnish, especially at the edges. If using caulk instead of epoxy, I would seal the tailboards before installing them.
IF you are not going for true restoration, I would consider using a thick plastic for the new inserts. Not only would this be more durable but - because these are Pintails - you could put a nice long, curved (but flexible) tail on the drakes that would hold up under use. Even the hens should have a nice pointed tail. (Photobucket is being cranky right now - otherwise I would post a photo.)
Hope this helps,
SJS