Depends on what's in there now!
Since the late 80's, the standard diesel engine in Catalina 30s was the Universal M25XP. I had one of these excellent engines in my 1987 model. It provided enough power to get my boat up to hull speed with some left over for head winds and waves (even with a folding two bladed prop). I was very happy with it until I sold the boat and bought a C-320. However, if your boat is currently equipped with an Atomic 4, the M25XP will require modifications to the engine bed to allow it to fit in the compartment and to line up with the prop shaft. There are at least two models of diesels that are advertised as direct drop-in replacements for the A-30. I believe that one of them is a Yanmar. From what I have read about diesel engines, nearly any modern marine diesel is very reliable and will give years of service (as long as it's not abused too badly). From this, I would probably purchase and install one of the "direct replacement" models. Yanmar engines have a reputation for being noisy. Based on my limited experience with my C-320 with the Yanmar 3GM30, I can affirm this reputation.
