Poor quality speakers? Thought it was originally a single 'normal quality speaker' under the dash or somewhere with no original cut outs. Dump it, put in the a modern head with bluetooth, blah, blah, couple of standalone speakers in the back, plenty of room, a proper original 70's mod, and only play 70's tunes, job done.
i would take your old ones to an audio center and get them to find you a replacement as theres so many types of speaker out there nowadays and each have different sizes and depths not to mention some are flush mount and non flush mount.
what kind of stereo do you have? original or aftermarket as they will need to know the impedance and rms wattage of the old speakers and ideally match them like for like.