You're right about the fact that speakers are discontinued after a while and old replacement models are no longer available. The way most of us have done it is to go to a place like Crutchfield.com where they specialize in doing this and know what is available and how to make it fit. You should replace the wiring and you have to be careful with the front speakers, the cones can't protrude or you have to make up spacers. You have to be real careful removing the rear quarter trim or it will rip. Use a heat gun on low to soften up the glue and slowly and carefully peel it loose after you remove the door seals. The original radios only had 8 watts per channel so if you are using the original radio you do not need to handle a lot of power. The original speaker cones have turned to dust and are not worth trying to repair. They were cheap pieces of junk anyway even way back then! Some have added a sub-woofer in the rear package shelf and an amp in the cubby. You can also put it under the seat. If you upgrade the radio the DIN adapter is the way to go.
David Teitelbaum