I just replaced some aftermarket speakers in my 123 with a new set that fit perfectly. Let me add that the 123 factory speakers are a metric size between 3-1/2" and 4". A couple of issues you often run into are the height and depth of the driver, as it can conflict with the speaker grille and the goodies below the dash, especially the air duct.
I found the Pioneer model TS-G1047, which is a 4" coaxial speaker for around $50/pair at local dealers such as Best Buy fit just fine. A nice feature they have is snap-off mounting ears which allow you to mount the speaker
using which ever set of ears are most convenient. They sound great, too.
As far as "brightening up" the sound in the forward cabin, I do this:
Buy a pair of small piezo tweeters, about $20 in most cases. Get a pair of 1" "L" brackets. Remove the inner door panels. Using the L brackets, mount each tweeter in the upper forward portion of the map pocket. Run the wires from the tweeter through the door hinge "accordion" to the dash speaker next to the door.
Still inside the map pocket, but pointing at the occupant of the seat it serves, the tweeters are stealthy but do a lot to brighten the sound in the front of the cabin.