The only problem is the missing ceiling, right? I wonder if it is posisble to sort that out with a bit of trial-and-error.Yes, if TOOL and GM Viewer won't touch the model file then it's likely to be unusable. Which indeed is a pity, as that room could have had uses in PoTC as well.
Extra GM files can be added using the "model.always" code. So if a model file can be made that looks like a ceiling, then it can be added without editing the base GM file.