Is getting Motaron's body part of the side quest for Jaheira. How do I get his body...I cant seem to get back inside the building once Xzar is dead. Can someone plz help

Actually, there's no use for the body, so don't stress. As far as anyone can tell it's simply a conversation piece.

According to slappy, it's possible to stop the assasin killing Xsar, maybe you can only get the body if you do that, I don't know.

No, the body will show up in a chest in the Harper's place at a later time...