When you are working with messages in the message store, you generally use the standard Object API objects. The Object exposes all the properties you need. The Task object is documented here:
The Body property is exposed on the Message Object type (Task is a sub-class of Message). Message is documented here:
Note you can only change the body and priority of Posted items – these properties are going to be read-only on a non-poseted item.
I hope this helps.