If it comes from the factory with a filter, K&N makes a better one for it. Doesn't really matter what you have or need, K&N will have it. K&N started out as a small family business ad is now a global titan of the filter industry, manufacturing high-quality air filters, oil filters, cabin air filters, and whatever other filter you could possibly need on a moving vehicle.
Most importantly, K&N makes an excellent line of cold air intakes for the Mustang and probably every other car on the planet. Many of these intakes are also 50 state legal and meet CARB guidelines for open air filter systems. If you don't want to make the jump to a full cold air system, then K&N has you covered with drop-in filters for your factory air box.