When I had my buggy, I bought a train horn for it. You need to get an air tank to mount in the bed, power shot tank is what I used. Rig up a hold open release shut valve in the air line where it runs into the cab. 120 psi is so much greater than the 10-20 the little compressor that comes with it puts out. I had people jumping for cover thinking there was a train coming at them. Funny thing was, the closet track was 40 or so miles away.
Test it in your carage by using a blower nozzel from your air compressor for your tools. Once you hear it, you will figure out you need more air.