There are no "direct" replacements for the existing OEM speakes. This link on YouTube shows you how a user replaced the existing speakers with a standard coaxial speaker such as an Alpine (similar to the Polk's).
http://www.youtube.com/watch?v=W0LPyCoq5k8
I did this upgrade and it is an improvement from the stock speaker but no coaxial will have the sound quality of a good component speaker witt a separate crossover.
I eventually replaced the speakers upfront with MB Quart components, ran new speaker wires to the front trunk area, wired up a new amp and ran all this off a new headunit. The improvement in sound quality from this was far greater than the improvement from just the coaxial speakers.
From reading around the favored top of the line component speakers are Focal, Hertz, JL Audio and more. Separates like Scan Speak and Morel are even better. All require proper amplification and a clean signal to maximize their quality.
Sound performance, like mechanical performance is usually a series of steps that all add to each other.