You have a Blue Box Athearn locomotive. Getting the shell off is easy. Since I cannot see a lug engaging the skirt on each side of the locomotive over the fuel tank, you must have the kind where you will squeeze the shell to disengage four tabs that go through the fuel tank. If this is correct, you will see four round holes and four rectangular holes on the bottom of the fuel tank. The rectangular holes are what holds the shell on. The round holes are the motor mounts. Just carefully push your fingers down behind the handrails above the fuel tank and squeeze the shell. Wiggle it a little, and the shell should come off without too much of a struggle
Some research on the internet will get the information you need regarding the types of couplers that will work. I switched to Sergent Engineering couplers, so I don’t recall the exact coupler that will solve your problem. But, once you get the loco apart, you should be able to see what you need. Maybe something as simple as a Kadee No. 5 will drop in.
Hope this helps,
DFF