The re/tuning conversion will depend entirely on how complicated is the engine brand, in some same CC engine blocks will need to change to next carb to match next HP top model, in others just main jets, air gaskets, etc a much cheaper option. A matter to have the spare parts list for that engine brand model and have a look to see which one applies. Won't be able to get a Tohatsu 3.5 to it's max, as it's alreday at the max compared to it's 2.5 carb restricted younger brother. Contact a skilled outboard tecnician.
Tohatsu also makes same HP engines for Nissan models and portable ones for Mariner/Mercury in the range : 30 HP (4 strokes) 50 HP (2 strokes) models.