Nearly two decades ago, journalist Will Crutchfield uncovered a stack of moldy old manuscript papers filled with musical jumbles, abandoned compositions, and what appeared to be pieces of an unpublished opera by Donizetti. “It didn’t take long to discover the pages were in fact a lost opera. But it took nearly two decades, and return trips to Europe, to piece together a version capable of being produced. On July 17, that big step in this ongoing detective story will be taken at the Caramoor Center for Music and the Arts in Katonah, N.Y., when Crutchfield himself will conduct Elisabeth, the never-produced Donizetti opera he rediscovered.