To completely replace a valve you need to remove the inner portion and replace it with the new piece. The Doughnut Patch technique cleanly allows you to do that. See the linked "How to Replace a Raft Valve" for complete instructions.
This is an air-holding patch, so we strongly recommend using the combination of Clifton Hypalon Adhesive and Clifton Hypalon Accelerator for the job. This is the combo we use to build our Pennel Orca inflatables.
Patch diameter is 8 3/8" and the inner hole is 1 1/4".
If your boat has another type of material, make your own doughnut patch from that material and use the proper adhesive.