At least two of us on this forum have put a CS1600 PID controller (#80062) in our Austin XL with great success.
You may want to take a look at the #80061 controller for the CS1000 & CS1250 (& maybe the CS850), since those would have a more similar barrel size as your Navigator 850. It might have a different algorithm than the #80062 that would be better suited for your pellet grill.
If you've already got your hands on a CS1600 controller, go ahead and give it a try. The connections/wire harnesses should be the same. It might work just fine. But if you haven't bought the PID controller yet, I'd try the one designed for the smaller grills.