I searched around for the loudest replacement horn that I could find, and stumbled across the Stebel Nautilus electro pneumatic air horn. This thing puts out 139 dB! It is a compact air horn, with the twin horns attached directly to the compressor.

I studied the horn's dimensions, and measured the space available and figured it would fit. When it arrived from TwistedThrottle, one of the few places that sell it. (I have no affiliation with them, or with Stebel.) I saw that it certainly was compact, but it looked as though it was going to be tricky to mount.

After making several cardboard templates I designed and built an aluminum bracket that works perfectly. I have included below several photos of the horn, the bracket and the completed installation.

The horn is mounted to the same chassis bracket as the stock horn, but needs a 3-piece bracket to hold it in the right place. I added a 1/4" plastic tube to the air intake so it wouldn't ingest water in the rain, as suggested by the tiny instruction sheet included with the horn. I also covered up the electrical connections by using the plastic top of a spray paint can.

So, just how much louder is it, you ask?

I measured the sound pressure level of the stock horn and the Stebel, and can report that the Stebel is 10 dB louder, which is quite a bit louder.  To get a subjective appreciation of just what this means, I have included the comparison sound file here. The first horn is the stock horn, the second is the new Stebel. These were recorded under the same conditions.

Conclusion
It took a fair amount of time to mount this horn, but the results were certainly worth it.