Mark,
Since your boat is now 30 years old, it will most likely be impossible to match the color of the plastic frame of the originals since they have yellowed over the years a bit. I do know that they make different depths to the flanges to match the cabin thickness. You could just replace that last portlight to match what your marina installed on that particular side so at least it looks the same and not many people would pick up that the two sides are different, of course you would and that could be an issue if you're like me
The next option would be to use a white paint that adheres to the plastic and paint all of the originals to match the new bright white ports. Krylon makes a white spray paint that could work. Rustoleum may as well. I used the Krylon on my plastic compass cover several years ago and it still looks great adn has stayed true white.
Eventually, you'll want to replace all of them because the plastic dogs/flanges get brittle with age and break.