Rhino liner vynil top

You are correct, you won't find a vinyl top in a catalog for your wagon.
Tops can be made for it though.
I have made and installed more than I can remember for long roofs
such as limousines. They are custom fit. They are much more difficult since they are padded.
Your wagon would not use padding if keeping it the way it was done in 68.
This makes the install much easier and can be done by most enthusiasts that are
hands on if careful. Getting the seams located proper and straight is the most
challenging part that makes or breaks a good job. Those would be some long seams.
Finding a shop that will make your top is the hard part. They would need to be local
so your wagon could be used to make the pattern since there is none.
There is a chance you could measure the distance between the seams on a b-body to get
a reference or start.
Painting a simulated top on would likely be a lower cost and easier plan for you but if you
did want a real top it could be done if you didn't mind the extra effort needed getting one custom made.