is there any sorta thing as a rectangular sub that can replace the sub that came standard on my car?

 

there are rectangular subs, but they aren't going to fit. the speaker casing is circular

 

The Kicker L7's are square, but they probably wouldn't fit in place of the stock sub and they need an enclosure rather than the IB application (they are IB right?) in the rear deck.

If you do replace the sub, sound deaden the rear deck very well, sealing the trunk off from the car; this will help alot with bass response.