Measure the hole pattern of the mounting screws and draw a simple plan. Go to your local audio shop and see what they have that fits. My E30 is 140mm on the diagnal, with a 4" cone. Found speakers with 5" cone and co-axial tweeter that was a drop in fit.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i