The Alliterative Morte Arthure survives in one manuscript, Lincoln, Cathedral Library, MS 91, one of Robert Thornton’s manuscripts. The picture on this page is based on an illustration found opposite the opening of the text.
The illustration looks to be a later, amateur effort (see Thomas Crofts, “The Occasion of the Morte Arthure: Textual History and Marginal Decoration in the Thornton MS,” Arthuriana 20.2 (2010): 5-27)
And speaking of amateur efforts, this page is a repository for the sound files I made while reading the poem aloud for an undergraduate Arthurian literature class conducted online in the time of COVID. The goal was to get the whole poem out quickly, with an ear to dramatic emphasis as an aid to reading comprehension. That means there are some stumbles in the files, and the vowels were not carefully marked ahead of time: this is an approximation, but I hope might be useful.
The shorter sections all have an introduction, in modern English, that briefly sketches the plot points in the Middle English lines to follow. The long file is the whole poem in Middle English, without the introductions.
I would suggest downloading the files (which you can do from the three dots to the right of each player below); they don’t seem to stream particularly well online. They are .mp3s, so should play on a wide variety of devices.