Bear with the "longish" answer: Caterpillar had some vendor problems with the original 3126 with soft valves and soft block metallurgy. Then neither the 3116 or the 3126 could easily be made emissions compliant, even though their unit injection system was superb and extremely efficient. To "fix" both issues, Cat re-engineered parts of the 3126 in about 2002-3 and de-tuned it from 420 hp to 350 hp. The 2003 & up 350 hp 3126 engines are bullet proof and had none of the earlier issues. The engine's fuel system was still was not as manageable as it needed to be to meet environmental guidelines so it was replaced with a fully electronic common rail engine, the C-7. The last spec sheep I saw on the C-7 marine engines showed them to be available in 3 hp ratings: 400 hp, 450 hp, and 500 hp. THe only way to tell for sure what C-7 is in the boat you are considering it to look at the engine data plate which is usually on the valve cover. A wild guess is that the replacement was the 400 hp since it would be a drop in and probably wouldn't even need to have the prop pitch adjusted. You would need an engine survey by a Cat dealer with the ability to search Cat records and to plug into the ECM to scan the data record for codes thrown in the past. The big thing on all diesel engines is proper maintenance done at the appropriate and recommended interval, The ECM data may tell you if the engine has had a temperature or other critical alarm code triggered. The owner's service record is about the only way to tell if the maintenance was done and when. With out that info, you just have to rely on the engine survey for guidance……which is why keeping a detailed service record is so important if you ever plan to sell your boat. Engine oil samples are also critical to evaluating engine health. The samples cannot be done on new oil and must be drawn after and engine has been run at operating temperature under a load and must be pulled from about the middle of the oil level in the pan, not the bottom against the pan and not the top near the surface. Hope that answers your questions………….