Good Luck, It will be a hard thing to do. The subs that are in there are meant for that enclosure. Also, I am not 100% sure on what impedance is on the subs in there now. Also your amps probably do not have enough power to push any aftermarket sub, except for maybe a low quality one.

Your best bet is to either replace everything, or just keep what you got. It sounds good in there now, if it were me I would just leave it be.