SPRING VALE - The engagement of Margaret R. Troop of Taylor, Mich,, to Rudi E. Grzic of Sterling Heights, Mich., is being announced.Miss Troop is the daughter of Mary A. Stair of 43 Prospect St. and the late Dr. Donald E. Troop. She graduated from Sanford High School in 1976 and is employed as a bookkeeper by the law firm of Smith and Quinn, Taylor.Her fiance is the son of Mr. and Mrs. Ramano Grzic of Sterling Heights. He is a graduate of Osborne High School, Detroit, and attended Macomb Community College for two years. He is employed as a tool maker by the Drescher Tool Co.The wedding will take place Oct. 11 in Michigan.