What is the ruling on withdrawing from an engagement because the girl has been afflicted by magic for a long time, knowing that she is very attached to the suitor?

An engagement is a promise of marriage that either party may break at any time. However, it should not be broken without a valid reason. You have the right to break off the engagement, but if the girl is religious and good, do not rush. The treatment for magic is readily available through Islamic ruqyah (exorcism/recitation).