Autoharp seems to have got there first - while Beethoven might well have written more symphonies had he lived longer, the fragmentary sketches assembled by Barry Cooper don't necessarily have anything to do with one another, so this isn't at all on the same kind of level of "validity" as the realisations of Mahler's 10th by Deryck Cooke and numerous others, or Newbould's version (or for that matter Berio's) of Schubert's 10th. I did hear this piece once and thought it sounded like a well-done pastiche in the style of Beethoven, which is more or less what it is.
Here's a bit more from Professor Cooper:
Altogether there are around 250 bars of sketches for the first movement. Some duplicate or contradict each other, leaving less than 200 usable; but many of these can be used more than once, by means of repetitions and reprises such as occur in all of Beethoven's symphonies (for example, a theme sketched for the exposition will recur in the recapitulation). The sketches thereby provide us with well over 300 bars, while the remaining 200 bars or so (out of 531) have had to be adapted from the same basic themes by various means (e.g. by transposition and sequence) and developed in the way Beethoven normally did. Thus, all the basic thematic material is Beethoven's; but appropriate harmony has had to be added in places where it is missing, the movement has had to be orchestrated in Beethoven's style (with the aid of only a few clues in the sketches), and linking passages based on Beethoven's themes have been inserted where necessary.
The result is obviously not exactly what Beethoven would have written, and in certain places in particular he would probably have been more imaginative.
Probably??
