Merc 9.9 to 15 Horse Power
FYI: After studying the rumour for a few months that it can be done, it can.
Non four stroke 1996-2003 9.9 and 15 HP Mercury (Tohatsu designed/built) engines are the same with a couple of differences.
One: carburetion; Two: exhaust baffle. Possibly Prop pitch and Plug type.
Ordered the 15HP carb, The external carb Body is exactly the same, inside the throat is much-much bigger as are the venture. The exhaust tube ID is reported to be only 1/8th inch bigger so will not change it. Will lose "1/4-1/2 HP" not changing the tube which saves pulling the engine apart.
Don't need speed, just more power out of the hole-shot for faster plane since carrying two batteries, one a 31 series 108 lbs. What I like most is no increased engine weight.