I have worked on a few Eskas and Clintons, though not recently.  What I recall is either no pump at all, with water circulated from a pickup tube behind the prop, or impellers like the one in the image above.   The Eska and Clinton legs are very similar but I don’t know whether the impellers are interchangeable.

Clinton faded away in the 80s.   Eska shut down around the same time.  Most of these motors, at least the Eskas, were sold by Sears under Gamefisher or Ted Williams brands.  The Sears parts operation is a possible source if still operating.  (The stores are pretty much gone.)  The Eska powerheads were all made by Tecumseh, Clinton made their own.  Some Tecumseh lawn mower engine ignition and carb stuff may be adaptable to outboards.