I had to replace mine as well. A small crack in the fitting was spraying water back behind the cabinets when I had the pump running. If the spray hadn't shot up, over and into the cabinet spaces I might not have noticed the problem. I also heard the water pump cycle. (I'm hoping that water back there eventually ended up running to the bilge.?????)
I bought the replacement from West Marine thinking that it would be a drop in. As you mentioned it was larger in diameter and some cutting was required. Old hole was 3-1/4 dia. New hole has to be 3-1/2. I bought a hole saw understanding fully how difficult it would be to cut a hole without a "pilot". Went to work thinking I'd be able to solve the problem some how. My boat was getting shrink wrapped the next day and I wanted to get it done.
Sure enough if you try to start the hole saw, even at an extreme angle, it is a bucking bronco. Even if you run the drill backwards it was hard to control. I needed something to act as a guide. Then I screwed the four mounting screws back into their old holes, leaving them standing proud by 1/4" or so. The old screw hole pattern was just the right diameter to keep the new hole saw under control and drilling the new hole went pretty easily. It was just sheer luck, but I was really tickled that it solved the problem.
New fitting went in nicely with a new hole pattern for their new screws. Looks nice on the boat as well. I got the "chrome" finish. The old one was faded creme or off white in color.
Another question: I hadn't thought of it but my fitting failed last spring on one of my first outings. Was it a winter freeze issue? Is there a way to winterize it? I'll definitely have it on the spring start up list as something to check. Not an easy check. Besides the spray water under the cabinets, I also heard the water pump cycle. I've never had the opportunity or need to use the shore water hook up. I just wanted to fix it. (I had disconnected the hose and plugged it all last season.) I understand there are some unpleasant safety issues with the shore hook-up.