Wm Watt wrote:
Cutting a piece out of the middle of the hull should be no great
challenge. Just be careful to choose the locations of the joined halves
that they match exaclty.

How many boats have you seen that have the exact same hull profile
forward and aft of the centerline? Not many are designed that way.

Then sand off the inner surface around the cut
and apply a couple of layers of fibreglass soaked in resin through the
cockpit. Boat builders call it a "butt" join. I've used it in plywood
boats. While the boat is in pieces it should be easy to prepare the
inside, move the foot braces, etc.

That wouldn't be anywhere near strong enough. To do it right, you need
the inner glass, but you also need to feather the joint back a couple of
inches on the outside, lay in new glass to build the joint up to the
original thickness, then apply new gelcoat.

The challenge would come in enlarging the cockpit. I don't see how you
can cut a piece out of the middle of a kayak without effecting the
cockpit, and they tend to have moulded coamings to keep out the water
and to allow a spray skirt to be tied on. Before cutting I'd want to
figure out what to do about the cockpit.

The only feasible thing to do would be to remove the coaming first, then
enlarge the cockpit opening to the proper size after the boat is
shortened. If the cockpit is recessed or the shape of the deck changes
significantly, it will take a LOT of work to reinstall the coaming.

The bottom line is that this project simply isn't practical.