Question?
Is it a Piston or Jet? because it or parts may be available through Micro-Mark. They are on page 5 of the Christmas Catalogue. Across the Bottom of the page. If it is Renwal's Piston engine all I can recommend is to replace the missing part with another from the pieces .
If it's the jet you might get by with the one I have suggested. Now, That said I do know the Renwal may be the older jet type .If that is the case you may have to make the part based on the drawings in the instructions.
There is another source though . The Smithsonian has a kit out, Hard to find, of a jet. This may be the one you need. I hope this helps and Welcome here. This is what we do here, help each other build better models! T.B.