Need to replace engine (rod
If you are going to put a used engine in the vehicle you will want to make sure that the engine is no more than one year before or after the date of your original engine so that you will not have to change very much from the old engine over to the engine that will be going into your vehicle. The auto industry changes their vehicles every year, and the more years that are between the original engine and the one that is going into the vehicle will often translate into how much has to be removed from the old engine and installed on the new engine for it to properly function in that vehicle, and especially to make the OBD-II system happy so that there is no check engine light on after installing the engine, and the difference can be from just having to change a sensor or two, to having to change all of the sensors, the fuel injectors, the oil pan, intake manifold and or throttle body, the exhaust manifolds and the valve covers. The most important thing is to be certain that all of the sensors that are on the old engine are exactly the same as on the engine that is going into the vehicle, and if any are different then swap them with the ones from the old engine. There should be no more or no less sensors than the original engine had on it and all of the sensors should be exactly like the original sensors or they should be the original sensors that are on the engine when the engine is installed into the vehicle.
There are a few things to check for when purchasing a used engine, and check the tune-up parts to see how well the engine was maintained, and check the spark plugs to see how each spark plug was burning and that gives you an idea how well that cylinder was functioning, and what you are looking for will be uniformity among the spark plugs, and optimally there should only be a light tan color to the spark plugs, and any spark plug that is not or is fouled out with carbon or oil fouled was not burning properly and that cylinder will have had some kind of either mechanical, electrical, or a fuel related problem with it. Also remove the oil filter if it has one and pour it into a container or a drain pan so that you can look for any fine brass clouds in the oil, or for any metal shavings.
If you are purchasing a new or a rebuilt engine then it will not matter because you will have to change over all of the sensors to the new engine, along with the intake manifold, the valve covers, any exhaust manifolds and usually the oil pan.