Last year I made the most interesting discovery that in the month his violin concerto had it's first performance (March/April 1845) Mendelssohn was working on a 6th symphony in C-Major! For the 1st mov. he wrote the first about 80 bars in full score and some sketches for the 2nd theme and the development. This fascinated me so much that I tried to make a full movement of it.
I finally could also get the sketches for a second movement from a library in Oxford and made it to complete also this to a whole Andante movement of about 14 minutes.
The first 2:50 minutes are based on a full melody sketch with various hints to the accompaniement by Mendelssohn. After orchestrating this I continued with a new middle part with various themes with each increasing in strength and coming to a dramatic climax with an appassionate fugato.
After this various variations of Mendelssohn's beautiful Andante theme follow until a recapitulation of the beginning and a most tender final variation.
I.Allegro
http://www.gerdprengel.de/Mendelssohn_unfinished_symphony1.mp3
http://www.gerdprengel.de/Symphony_in_C_Allegro.pdf
II. Andante cantabile
http://www.gerdprengel.de/Mendelssohn_unfinished_symphony_Andante.mp3
http://www.gerdprengel.de/Mendelssohn_unfinished_symphony_Andante.pdf
I can't speak to the authenticity of the piece as I don't know it, but I really enjoyed listening to it, great work!