Since this is just a dream thread I'll contribute some thoughts. A diesel should be eight wheeled and should be either diesel hydraulic or diesel electric drive. The weight on each truck should be around 22,000 pounds for a total of 44,000 pounds , and it should have a large enough power unit to give it 11,000 pounds tractive effort. That is about twice the tractive effort that #9 is supposed to have and about the same #6 was supposed to have had, with similar axel loading. This would allow the diesel to rescue any stranded trains and return with them.
Now one possiblity is to buy trucks from the Australian cane locomotives instead of the whole locomotive and use a new gen set made here. The frame could be shop built. I would favor a boxcab body, as it would be easy to build, roomy, and typical of early diesels.
Mike Nix