Q: Engine swap, would a Mazda 3 2.0 fit in a Mazda 3 2.3 - I don't know much about cars, but I need a new engine

The hardware and software of the computer (powertrain control module) and transmission control module (TCM) in the Mazda 3 will only work with your present engine. If you change the engine to one that is not a catalog interchange for the car you will have to carry over the "new" engine’s computer, TCM and complete vehicle wiring harness in order to get the new engine to even start, let alone run. Unfortunately, with modern cars, there is no practical possibility for using non interchange engines unless your intent is to completely re-design the vehicle. If your engine has failed, and thus you simply need a replacement used engine, please note that you will easily find many direct interchanges using on-line used parts databases such as Car-Part.com. Finally, if you are interested in an engine replacement (OEM to OEM), YourMechanic professionals in certain locales can assist with that. Please simply inquire based on your locale. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.