That's a good question that is multi faceted. Best asked
here methinks.
Sometimes an all in one is not available so you have to go custom. And make sure your tower will accommodate the parts. Rad/reservoir etc.
You will probably find everything you need
here. As long as the Myst is based on the reference design. I haven't looked into it myself.
Edit: Here is a pic of the
Myst. And I think a
reference 7870.
Edit#2: Confirmed, the pics in the second link are indeed reference. So it looks like your good to go.