I think you are corect that you would have to make changes in the MDL file.
The only problem is in order to do that you need the orignal source file that was used to make the MDL file to start with and some designers do not like handing there files to just anyone.
And there are several programs out there that one can use to design qircraft with GMAX is one but there are others as well and not all are compatable with each other. BY that I mean you might have the same program that was used to make the plane with. GMAX is able to import some files but I am not sure if it will import all of them.
It would be easier to start from scratch and redesign it on your own. ALTHOUGH THE WORD EASIER does not imply that it would be a easy task by no means
