It was Mozart's friend Gottfried Van Swieten who fired his interest in the music of the Baroque period, especially Handel's, commissioning additional orchestrations for several major works, including Messiah (1789). To Handel's relatively small instrumental complement of oboes and strings, plus occasional trumpets and timpani, Mozart adds pairs of flutes, clarinets, bassoons and horns, plus three trombones, while expanding the role of the timpani. These days we regard Handel as not needing such expansion, but throughout the 19th century this version was commonly performed.
Perhaps we should regard this arranged version as Mozart's commentary on Handel, or as a large-scale variation. In any case, this is not hack-work but an appealing tribute that translates the music's idiom from Baroque to Classical, with many graceful and imaginative touches.
This 1988 recording, though not in the clearest sound, benefits from what was arguably the finest quartet of soloists then available, together with the confident participation of the Huddersfield Choral Society and the disciplined Royal Philharmonic Orchestra which moves efficiently and purposefully under the direction of Sir Charles Mackerras, a noted specialist in both composers.
The work is sung in the English Handel set rather than the German of Mozart's Viennese performance. Mackerras also gives those arias reassigned by Mozart to other voices back to those Handel originally intended and he also makes standard cuts in Parts II and III. This won't replace an all-Handel venture, such as the excellent one by William Christie, but by way of comparison it's highly enjoyable.