@Hiro Chen what is the actual purpose of your question? Is it a general conceptual thing or related to a specific design?
And I don't get the point why the engine or propulsion concept effects the CE Design Category, which covers stability and seaworthiness. Yes, the engine it self must comply to some emission regulatories, etc.
You wrote that you want to convince your boss not to use such a concept, but it also has some advantages.
For example it is way lighter than a inboard diesel engine with the same power output.
The Seascape 27 is constructed in a way that you can take out the engine, with a folding mechanism, during sailing to reduce the drag.
The engine might be more easy to maintain by a better accessebility.
Once there was an outboard engine converted to an saildrive on the market, but I guess it is not available anymore, unfortunately. You can Google for "Saildrive 330".