Well, I guess I would so some basic checks before doing any major work. No sense rebuilding the carb on an engine with low/uneven compression or a gearcase full of water….Check to see if it has spark on all four cylinders, you may need to clean and readjust the points. Then I would try to start the engine to see how/if it runs.
From the looks of the picture you posted, it would appear that the engine was carefully maintained. The carb may not need cleaning/rebuilding if in fact it was actually drained prior to being stored.
If the engine seems to run OK, then I would pull the gearcase and replace the pump/impeller. But again, be sure to drain/inspect the gear lube first. Milky lube, or raw water draining out spells trouble. In that case, once the gearcase is removed it should be pressure tested to pinpoint leakage issues.
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