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It sounds like your motor is mounted using the "blind holes" in the engine bracket.

Yes, the engine bracket has threaded holes for this called blind holes.
This is a very common problem for this vintage of Whalers.
Check out this article and drawing about the blind holes.
http://www.whalercentral.com/articles...icle_id=82

Before raising the engine which will require 2 new lower holes and filling the old blind holes, try a different prop and see if that gets you anywhere close to the top speed you are looking for.