Here is a link to a post about a
Bonneville Honda Civic that topped 200 MPH. As you would guess, there is absolutely nothing stock about the engine, but there are pix of the front and back end showing some fairly conventional, but apparently effective, aero mods in addition to an under body pan. Take a look at the
rear wing. It looks to me to be a reasonably easy to duplicate design.
[Edit] P.S. Here is an image of another
shelf spoiler of a Bonneville Civic