I bought a copy of the sheet music for this piece then input the notes into Sibelius. A MIDI file was then exported from Sibelius and imported into Reason. From there I added a couple of effects and tweaked the MIDI data a>
All of Mozart's music:
http://www.mozart-archiv.de/
All of Mozart's scores:
http://dme.mozarteum.at/DME/nma/nmapub_srch.php
Lacrimosa dies illa
Qua resurget ex favilla
Judicandus homo reus.
Huic ergo parce, De>