For this version, just the trumpet and trombone parts have been reduced.
Nothing could be gained by reducing Mozart's scoring of basset horns, bassoons, timpani and strings for this great masterpiece.
There are, however, a couple of options open to you.
The Basset Horn parts can be played perfectly well on clarinets. Mozart's scoring never goes below the range of the clarinet and published sets of orchestral parts normally include parts for basset horns and for clarinets.
The original 2 trumpets and 3 trombones are replaced by 1 trumpet and 1 trombone. The trumpet is needed to retain the colour in Mozart's scoring and the trombone is essential in the Tuba mirum.
The full score is not provided. I just hire out the trumpet and trombone parts.