The work had been considered lost. It was found in the 20th century, then disappeared again. Until recently.
Mace made the discovery after locating and examining a manuscript of the piece in a private collection. She also determined that it had been written in 1828, and not 1829.
The piece is in Fanny Mendelssohn Hensel’s handwriting, and also contains stylistic elements that indicate that she is the composer. 
The above is a beautifully illustrated manuscript of Morgengruss by Fanny Hensel, courtesy of RISM, and below is a performance of a slightly different version of the piece. RISM holds a number of Hensel manuscripts, some of which have been digitized and are freely available online (look for the blue bar with an e to the right of the entry with “Online lesen”).
And finally, here is Fanny Mendelssohn Hensel’s Quartet in E-flat major
Image attributions: Portrait of Fanny Mendelssohn Hensel by Moritz Daniel Oppenheim [Public domain], via Wikimedia Commons https://commons.wikimedia.org/wiki/File%3AFanny_Hensel_1842.jpg
Morgengruss manuscript image courtesy of RISM under Creative Commons License (CC BY-NC-SA 4.0). http://www.rism.info/en/home/newsdetails/article/2/fanny-hensel-1805-1847.html