Hi, Wyowa. The Hardin is very Prill-like. If you are in Sheridan, you can stop by Prill Brothers. These are the same people who manufactured the Prills 30 years ago, but it is a different business now. They have a lot of parts laying around and some expertise if you catch the right person.
If you are getting coal in Sheridan, either at Schultz or Mullinax, you'll be getting sub-bit coal from Decker, MT. It's a lot higher BTUs and much cleaner than what you can get from the Powder River Basin. I think you wouldn't have a problem burning Decker or PRB coal in your Hardin, though I agree that you'll have to do some tinkering to accommodate the different qualities. You might want to consider the extra 20 mile drive to the retail yard at Decker- Wolf Mountain Coal, which is generally cheaper than Mullinax or Schultz. PM me if you need phone numbers.