Decreasing engine box size

[thinwater]

When I replaced a Bukh 20 with a Yanmar 3GM30F it left a large space above the engine. I made a tool box that slid into place on aluminium runners. No consequences for the engine.

This is the only suggestion that makes sense to me. Metal tools only. Could be handy.

You can never have too much space around an engine. I can't see the little bit of space you will gain being worth the downsides.

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Typically, the engine air intake is sufficient (combined with water cooling) to ventilate the compartment. Outboards are cooled with no fan pulling from the cowling, just the engine intake. Do make sure the louvers are large enough to admit enough engine air.

a. The free vent size (subtract the grate area) should be at least hp/3=square inches. (This is several times bigger than the engine air intake; you don't want the velocity that high or that much additional pressure drop. Industry rule of thumb.)
b. Typically, engine manufacturers state no more than 30F rise in the compartment. Check it.

I can't see removing a fan that works. If you ever have a diesel leak or other bilge stink, you will want it back!