You can upgrade a 9.9 to 15 on most 2 strokes engines if being same CC, some more easier and friendly to upgrade, others needs more parts change to match 15 HP versions. If the Mariner Super 15 it's same as the newer Merc Super 15 version, which are rebadged Tohatsu cowls, are 18 HP in reality. Fancy names, marketing issues...
Happy Boating
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