That is a very special Seagull, there are very few like that around.
The spark plug cover is, indeed, a weak area so I would recommend a replacement rubber one, readily available from "Saving Old Seagulls".
Beware, the external finish of the motor will rapidly deteriorate if constantly exposed to the salty elements. This will not cause any operational problems but you will almost certainly lose the current uniqueness of a pristine Seagull
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