If the combustion chamber is still in good shape (not rusted through), you can probably rebuild most of the furnace. Like Channing, I just rebuilt mine last month and its working great. Found all the parts at PDX RV, and for about $150 completely rebuilt my furnace. They also sent a service manual for my furnace, with all the instructions. Took less than a day to do all the work. I think the Suburban furnaces are made to be removable so you can work on them on a workbench instead of hunched over the side of a RV.
If thats not an option, how about looking at an Atwood replacement? Maybe there models fit your existing cutout better?