I have a mid 80s Johnson 15. These are about the biggest motors that will physically fit (and tilt up) in the opening. They are the same dimensions as the 9.9 motors, and frankly I'd rather have one of those. The boat would go just as fast and not use as much gas. After about 92, even the 2 strokes got too big to fit, because they put the recoil starter mechanism on top of the motor instead of beside the flywheel where it had been before. There are lots and lots of these older Johnson/Evinrudes around on the used market, and they are extremely dependable motors. I have used a wide variety of motors on my 27 and can give you first hand experience of what works and what doesn't, if you're interested.
I pulled a 15hp johnson sailmaster off my C27 about 8 years ago and it's still sitting in my garage waiting for me to rebuild it. I will give it to anyone who wants to drive out to riverside, ca to pick it up. It's an electric start model with no tiller, long shaft and it fits perfectly in the C27 motor well. Will throw in the tech manual as well. 951 218-9161