The good news is that the OEM system isn't pushing a whole lot of power so some lesser expensive speakers should last OK. Sound quality may not be the greatest but will probably not be much different than the OEM speakers (they weren't exactly high end).

The bad news is that it's quite difficult to actually remove the rear panels to get to the speakers to do the replacement ... doable, just difficult. Also, many of the aftermarket speakers of the right dimention end up being deeper so the back of the speaker ends up hitting the metal fender before the mount actually gets to being flush with the mounting surface. Your best bet is to remove one of the OEM speakers & take it with you to the sound shop (or WalMart, or Best Buy, or ...) & have the sales tech recommend which speaker to buy for size & fit. Just don't let them talk you into a better speaker than you can afford.