There is no point in replacing the engine until it is proven that something is causing that engine to heat up which can't be fixed.
Often when the radiator is refilled after a repair; there will be trapped air pockets in the cooling system. Air in the system will cause the engine to overheat and the temperature gauge to not register while the coolant is boiling. This can ruin a perfectly good engine. To prevent this from happening; after the system is first filled; only drive the car far enough for it to just warm up; then shut it off and let it cool down. Then remove the RADIATOR CAP when the motor is cold, and fill the radiator to the very top with a mixture of half coolant and half distilled water. Filling the plastic reservoir will not work when this happens; the radiator must be filled directly at the filler neck. Keep filling the radiator at the cap opening after every time the car is driven until the level stops dropping; or until it becomes clear that the engine is sucking coolant out of the radiator (usually through a blown head gasket). If the level stabilizes; you can check the coolant at the reservoir after that. If the level keeps dropping; the engine will have to be repaired or replaced.
Once an engine is allowed to badly overheat; the cylinder heads usually warp, and the head gaskets start leaking. After that happens; the engine will continue to overheat even if it is filled with coolant. It is too late after that point. The thing to do is to not let that happen by shutting it off if it begins to get hot (which you can check by opening the hood and feeling how much heat is coming from the engine, as well as looking at the coolant reservoir. NEVER LET AN ENGINE CONTINUE TO RUN WHEN THE COOLANT BOILS!!!!.
If you installed a used engine in the car; that engine could have been defective; or it just could have been that the radiator coolant level was not rechecked and refilled after the first time the car was driven. That is one very expensive mistake to make!!!!
If a 3.5 motor was installed to replace the original 3.2; the computer must also be changed to the computer from a 3.5 engined vehicle. The engine will not run right if this is not done; and it may overheat from improper fuel mixture and ignition timing settings caused by the wrong computer.
Many times after another computer is installed; it must be programmed with the vehicle ID number before the motor will run right.