modifying from source files

How hard would it be to modify the model of an aircraft if you had the source files?
I want to change a prop cri cri to a jet cri cri.
I know where to get the source files for gmax and I really want to see this done.
I want to change a prop cri cri to a jet cri cri.
I know where to get the source files for gmax and I really want to see this done.