There are some parts layout diagrams on several of the websites selling BMW parts...I'd start there.
Most any horn that ends up louder would probably draw more current. THat may or may not be an issue. Many of the circuits in modern BMWs, the switch is a low current device with a circuit board that tells the computer to turn on the thing...not like days of old where it was a real power switch, or to control a relay. The 12vdc battery on the i3 isn't huge, and they chose small current devices with their smaller wires to power them. Going bigger horn, may require more than just a simple straight replacement. Do not know for sure, but it's a possibility.