Manuscript Becker III.8.51, owned by the Universitätsbibliothek Leipzig, contains four fugues for organ or keyboard, from Friedrich Wilhelm Marpurg. They were copied from an unknown source by one J.A. Dröbs (if I read the insciption correctly). This Dröbs was not a very reliable scribe, as there are several obvious mistakes in the manuscript that is additionally often very difficult to read. The scores of these four fugues contain here and there some gueswork of what was actually meant and probably deviate in certain spots from Marpurg's original. As long as I do not have another source for these fugues however, this is for now the best I can do.
The third fugue is really a mess in the manuscript. The first problem is that the first statement of the fugue theme differs from the theme in the rest in the manuscript. Probably the scribe read some clefs wrongly in the original. To make some sense of the music I have to presume the tenor voice is notated a third too high in the manuscript for almost five bars. The second problem is that the fugue is probably meant as a five part fugue, but the sribe of the manuscript never realised this or he did not know how to properly notate it. Or perhaps he had trouble reading the original. Because the third problem is that writing a five part fugue perhaps was a bit beyond the compositorial skill of Marpurg. Lots of notes are doubled between the voices, making the music very difficult to read and awkward to play. And the fourth problem is that the scribe notates the music in very little space, crossing out notes here and there and generally making a blurry mess that is very hard to read.
The score is therefore my best guestimate of what Marpurg's intentions possibly were in this fugue. As it is a five part fugue (probably) in style antico, I though it best to play it with a strong plenum registration.
The recording was done with the Hauptwerk software and the sampleset, made by Sonus Paradisi, of the Janke organ in the Stadtkirche of Bückeburg (https://www.sonusparadisi.cz/en/organs/germany/buckeburg-janke-organ.html).
Score available here: http://partitura.org/index.php/friedrich-wilhelm-marpurg-fuga-in-a-moll-a-5