I don't have many MTH diesels, but my MTH DL109 is assembled in a way similar to the Weaver, above. A large central screw; you may have to spin the flywheel to "unscrew" the worm from the spur gear. Remove the body to do this.
The die-cast zinc can be gently bent back into place, at times. You may have to fab some parts - use brass - to replace and or reinforce the damaged original. Take your time. Brass is easy to use; it needs to be thick enough to be stiff, but not massive. .030" - .035" thickness is a good starting place. Good stuff to have around, in any case. Hobby Lobby even carries some sheets.
Not sure what issues you have had with JB Weld (not the only brand) epoxy, but I have found it to be a good material. It requires a sufficient contact area and a clean surface - in other words, if that truck has an oil or grease film on it, the epoxy won't stick. Wash the attachment areas with clean mineral spirits on a rag or swab.
Importantly, take your time. It's just a little machine and can be fixed.
Also, while the body is off, eyeball/snug down your boards and connections.