Some of those Fiamms draw a lot of current, from what I've been told, and some stock alternators may not put out enough juice.
I installed miniature airhorns on both the Harley and the Honda - they're very loud and easy on the electrical system. Wind up almost instantly, too; no significant delay in making da noise.
Must agree with the air horns idea. I ran them on my Norton in '74. Worked fine.