Challenged,
I just bought a pair of speaker grilles for the 3-speaker dash in my '70 Challnger. I am now in the same position as you.
The grilles I got have the original speakers with them but the cones are trashed. They're just frames. You're right; the magnets are small. I haven't started looking for speakers yet but I will be soon as I want to put the speakers in the dash before I put the dash in the car (full restoration project). It's been awhile since I've shopped for speakers but I remember seeing some not too long ago that I think will work. But electronic products change so much anymore, who knows.
I'll try to update you on what I find..
MoparMan