Most replacement filters can be easily fitted without the need for any major modifications to the engine bay.
Another modification would be to install a complete induction system. Replacing the original air box normally means a much bigger air filter can be used. In many cases the square/flat type can be replaced by a cone shaped unit, resulting in an increased surface area. Using a cone shaped filter, enclosed within an air box will result in a colder air supply and therefore better performance. Fitting a new cone shaped induction kit without enclosing it will result in a better, more aggressive sound, as the air is sucked in. However, you should remember that the temperature of the ait being used will be higher and therefore performance increases could be compromised.