Yes, it all needs to be matched, or near so.
If the existing motor moves the ship well, then repair it and move on.
You really do not need a turbo.
If you try to extract that 75 hp, then you are going to be ploughing water and the fuel consumption will increase, not because the motor is les efficient, but because the boat realy drinks fuel at the higher hull speeds.
If you just have top end scoring, then it should not be too expensive. You should be able to get oversizes and re-bore the barrels. Then a valve seat re-grind (or re-cut and grind if they are pitted), a new cylinder head gasket and you shuld be ok.
Yanmar spare parts are not TOO expensive (compared to Volvo that is).
Cost it out, and see.