if 'complete' is truly complete (it is not always, as in the case of 'complete Beethoven Sonatas' which leaves out the 3 Bonn sonatas-as do most publishers unfortunately) the look through the Henle contents for each of the volumes (believe there are 4) of the Mendelssohn complete works for piano series. One of those should have it included. unlikely to find it published as a separate piece (though I would check with Schott, and consisder Vienna Weiner Urtext, and possibly Konemann but that's a long shot). I always have to end up buying multi work volumes in order to get one single piece in particular I'm looking for.this seems to be the peters versionhttps://www.prestoclassical.co.uk/sm/7135834
Also I didn't know Henle's Beethoven Sonata collection was incomplete - I own both!