Complex sentence: In 1972 he found and led a church-sponsored brass band for young people--the Fairview Baptist Church Marching Band--which became a popular and eventually became known as the Dirty Dozen Brass Band.
Split into simpler sentences: In 1972 he found and led a church-sponsored brass band for young people--the Fairview Baptist Church Marching Band--which became popular. In later years the band became known as the Dirty Dozen Brass Band.