Consider two chants that are found in the Graduale Romanum that are often used to accompany the Imposition of Ashes on Ash Wednesday:
“Let us change our garments to sackcloth and ashes, let us fast and weep before the Lord, that our God, rich in mercy, might forgive us our sins.”
“Let us correct our faults which we have committed in ignorance, let us not be taken, unawares by the day of our death, looking in vain for leisure to repent. *Hear us, O Lord, and show us your mercy, for we have sinned against you. v. Help us, O God our Saviour; for the sake of your name, O Lord, set us free. *Hear us, O Lord…”
For the vernacular, you may refer to the music for Ash Wednesday in Fr. Samuel Webber’s “The Proper of the Mass for Sundays and Solemnities – Chants for the Roman Missal in English” (in my humble opinion, a staple for any choir serious about the intent of recovering the traditions for the Ordinary Form).