Following is my experience (learning), if you can come up with better approach please update thread (or DM me)
Few things come in way if you want to use OEM speakers (if you can find them and if they are relatively priced) - I couldn't source them at all.
1. There are no amps driving at those levels (around 25 watt)
2. Front speaker + dash speakers are in parallel so effectively you have 2 ohm speakers in front and 4 ohm speakers in back.
a. AMPs are expensive for something like that
b. AMPs have way more wattage than 25watt - you will blow speakers (if end up increasing volume)
c. AMPs are physically big, you will be hard pressed to find place to put them and if you are relocating AMP then might as well do speakers.
3. Better approach is find speakers which fit in place (but have bigger watt), there are options for front (still need to drill screw holes), but I couldn't find rear.
OR
Get headunit that can drive around 25/30 watt each and bypass OEM amp, keep using existing cables and speakers (speakers may still be bad from all the years though)