I replaced the original horn with a pair of used horns from a 2002 Subaru Forester. I chose them because of their small size and one has a high note and the other a low note. To install them I moved the mounting holes on the horns' stocks 3/4 of an inch inboard and then shortened the stocks accordingly. I fabricated a couple of angle brackets that mount the horns to the same place as the original. I also welded a couple of tabs on the new brackets so that the mounting bolts won't turn. That way you don't need a wrench to hold the heads of the bolts.
The horns can be repositioned on the stocks in 90 degree increments if desired. However, make sure that the adjusting screws do not turn at all. Also I used a relay which was more or less required in my installation because I rewired my bike using an ammeter. One advantage to this horn installation is that the return oil hose can be routed in a much more natural position without any sharp bends to the oil tank.
Getting to the horn is really not that difficult. You need to remove the rear wheel and then the rear fender. It should only take maybe 20 minutes to do that. Probably the hardest part of the whole deal is disconnecting the wires at the tail light assembly (wires could be delicate and possibly break in that area). It is pretty straight forward.
Here are a couple of pictures of my Subaru horn 'upgrade':
I hope this gives you some ideas,