I did this mod on Saturday. Used an Italian-made Stebel GP2 horn kit. Had compressor, bolts, horns, and tubing included in the kit.
Remove the front grille, remove the black plastic shroud, then remove the horn bracket and unplug.
Mount the trumpets to the existing bracket. Splice in spade-connectors, connect the compressor, then mount it to the underside of the lip next to the right-side trumpet.
No drilling required, and only one wire cut (I wanted to use the existing horn wiring harness, so I crimped spade connectors onto it).
Horn is WAY louder, and way more Italian.
P.S. I did this same mod on my F40 three years ago.