Depending on your time and skill level: Gasket match inlet and exhaust manifolds, fit 1/2 to 3/4" larger exhaust. Better flow but with out the "Please stop me, Officer' noise. Depending on funds consider a cam re-grind to a mild road profile. I'm sure your local shops or rev heads know who to speak to.
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