Although it may seem like a small detail, it is absolutely vital that the tinware and engine compartment rubber seals are all present and intact, in order to keep cool air above the engine and hot air below it. If tinware parts are missing, or the seals around the front and back of the engine are torn or broken, hot air will be drawn from the heads/exhausts back into the cool zone around the top of the engine and recirculated over the cylinders and heads, causing the temperature to rise, often to a critical level. This can cause all kinds of problems over time, some of which may not be immediately obvious, from hot starting troubles, to cracked cylinder heads, and up to and including a seized engine. Did we mention that this is important?
If you've just bought a car, it's well worth checking the condition of these parts and also making sure that there are no foreign bodies stuck in the cooling fan (do this with the engine turned off!).
If you are fitting a reconditioned or new engine, then don't just rely on refitting the parts that were on the old engine, as they may not be correct either. Check that everything is correct, matching and in place.
The thermostat is the other vital piece in the cooling system. The factory fitted a set of flaps inside the fan shroud, that actually block cooling air when the engine is cold, in order to warm up the engine more quickly. These are opened by the thermostat and obviously, if this part is defective, your engine will very quickly overheat. Check and replace if necessary. The alternative is to completely remove the thermostat and flaps, which while it certainly simplifies matters, is not ideal. It means that your engine may never reach the correct operating temperature in cold weather conditions, though this is far less serious than overheating.