My brother is a welder by trade but a quality inspector for American Honda. He says the quickest way to render something irreparable is to use JB Weld. A cutting torch can't even burn through it, making a proper weld repair impossible. If the horn is a beater (Chinese, Indian, red-rot special), JB Weld it and forget it. If it has any value, do as rowuk says and have it professionally repaired.