Mortise latch

Hey Bob. We often see the types of mortise locks compatible with the 82-086 in Jeld Wen doors, and so in theory it could work for you. But I'd like to see a picture of the mortise lock to be safe. Moreover, I'd like a photo of your existing handle set. The 82-086 has a thumb lever and key lock fixed in a lower offset position (i.e., it's closer to the bottom mounting screw than the top one). If your existing handle set has the thumb lever in a different position such as perfectly centered, then the 82-086 will not work for you. Photos will help us confirm your best course of action. 

Perfect, thanks! These photos are a huge help. Seeing your door, I do believe that our 82-086 can work on your door. Just note that the thumb lever is in a fixed position and cannot be adjusted. Your thumb lever appears to be in the bottom orientation, same as our 82-086, but it still may be something you want to check. If it's centered, instead, just let me know and we can look into other options.