Hello Ian, Happened to spend time at a swap meet on the weekend with an old work mate who specializes with Morris Minors and I happened to mention the fact that in the past I have filed oil filler off the early morrie caps as they fit the Doug tanks. He remarked that I had raised the subject before. So I said are they available and he commented yes but didn't really offer any. Now if you have a collector or Morrie supplier down you way may be worth a try. The brand of cap is Flavel and also the later model caps have the same two post clip, but a new top would have to be knurled and attached. The oil cap clip on air cooled VW motors is also useable.
Maybe a way out if you haven't been successful. regards Alan