Do you have stoker size bituminous coal in your area? Might make more sense to source a bit. stoker furnace. They come up several times per year on Craigslist.
e.g. **Broken Link(S) Removed**
I know it wasn't your goal to replace the entire unit, but it may end up being less expensive (with better results) to do that compared to a retrofitting the Newmac.