The A-4 on my C&C 34 blew last season. I decided to replace it with a rebuilt A-4 from Moyer. Moyer is a great company, helpful, honest, responsive. It really wasn't that difficult a job. I used the mainsheet to lift the old A-4 out. I put the halyard on the boom for extra support. I did not have to make any modifications to the boat - it really was a direct, bolt on replacement. Of course, it was a great opportunity to clean the bilge, check wiring, fuel lines, etc. The new one was fresh water cooled, so I had to locate and plumb the heat exchager - no big deal. It is also helpful to get one after market part - the extended dip stick. This makes your regular checking of oil level a breeze.
This spring, the new A-4 fired up in literally a second of cranking. I love these little engines. I also hate the smell of diesel. You can't be the price. A used A-4 can be found, usually for a couple of grand. Many, if not most, of the older ones were salt water cooled even if they are now fresh water cooled. I personally think it smarter to buy a rebuilt for 4500 with the piece of mind that comes with it.