Not a project I would take on...
They appear to have several sizes of scupper plugs and scuppers, but only list one size (medium) of this scupper "Valve". No sure why. Maybe never built the other sizes?
There is a chart of their suggested sizes here: http://www.oceankayak.com/uploadedFiles/Website_Library_Assets/Files/Product_Support/Scupper%20Size%20Chart%202012%20%282%29.pdf
Their plugs look to allow for a wide variation in scupper size and they have many sizes. The valve appears to have a very tight tolerance. I can't see it working in many sized scuppers.
I have no comparison data of scupper dimensions for our boats.