Joseph Smith never claimed he translated the Book of Abraham from the fragments that remained and were found in the NY Museum. He had in his possession much more that was eventually sold and then burned in the Chicago Museum which burned in 1871.
"After the Latter-day Saints left Nauvoo, the Egyptian artifacts remained behind. Joseph Smith’s family sold the papyri and the mummies in 1856. The papyri were divided up and sold to various parties; historians believe that most were destroyed in the Great Chicago Fire of 1871. Ten papyrus fragments once in Joseph Smith’s possession ended up in the Metropolitan Museum of Art in New York City.
24 In 1967, the museum transferred these fragments to the Church, which subsequently published them in the Church’s magazine, the Improvement Era.
25"
from Translation and Historicity of the Book of Abraham