I have a 9.9 Yamaha, which from everything I have seen is the same as the mariner. Mine is an 1987, and what I would need to change to a 15 is the carb and the reed cage assembly. Not sure if the older motors have this though, as I believe they changed in 1984.
I would sell your motor and just buy a 15hp OMC motor around the same year as your current motor.
Yamaha's bring a premium in price so you might be able to sell your motor and buy a 15hp OMC and not be out any money.
OMC motors are just as reliable as the Yamaha's. Parts are considerably cheaper and easy to find used if need be.