One of the faults with the 1300, just as what's found with many makes & models of bikes, is its very ineffective stock horn.  It should be a serious "attention getter" once you push that horn button.  Here's what many are using as replacements:

**Stebel Nautilus Compact Air Horn - This requires an inline relay ($10) to prevent the horn button from being damaged. 

**Wolo Bad air horn - (a repackaged Stebel - the very same horn - see above)

**Fiamm Freeway Blaster(s) - either as a single horn, or as dual horns (low & high tone).  One Fiamm is simply a swap, whereas using the dual Freeway Blasters also requires an inline relay.
**Mini Beast air horn - a Stebel Nautilus packaged in a superb looking housing that looks and fits well on the left side of the engine in place of the empty plasti-chrome V-cover (requires a relay).

There are obviously other choices, but those listed above seem to be the most popular of the bunch.

RELAY INFO (provided by MaiKai):
http://www.bcae1.com/relays.htm

and 
http://www.installdr.com/techdocs/999404.pdf