Do you not recall replacing the cheap plastic valves on Udisco boats right out of the box?
1) Remove old valve
2) Use reinforcing brass plate of new Brass Valve to mark inside and outside circle
3) Cut inside circle. Carefully. Very carefully.
4) CAREFULLY work base of new valve into boat; In every circumstance is it unacceptable to cut or tear the hole larger than the base. As Bonzo said once; slow and easy gets er done, rammin and jammin is for amateurs.
5) Using liberal amounts of glue etc. create and install a circle patch to reinforce cheap Udisco Hypalon area around new valve.
6) Install outside reinforcing brass plate.
7) Crank the F^*k@* down using really big Vice Grips (note; you are not allowed to scratch or dent the brass nut, base plate, or valve body).
Blow up boat; check for leaks
9) Drink celebratory beer.