Rob,
I suggest you study the prime directive (as I call it) of the WW&FRR. James summarized it rather succinctly. It takes a while to sink in, but the railroad is not just a touristy operation, but is recreating history as closely as possible. The internal combustion locomotives are a necessity for convenience, but steam is used as much as possible. There always are concessions to moderism and safety, but these are meant to be kept to a minimum. This is why #9 is being repaired for operation, and the next build is #11. In terms of the philosophy of the railroad, a modern center cab diesel just doesn't fit in.
Notwithstanding what I have said above, it is fun to dream and it would be interesting to see what you have in mind. I see a stretched version of the GE on the MNGRR. I always felt they were awkward looking locomotives and that a redesign was in order (wider cab for one)(body mounted couplers for two).
I have been involved in the #11 project, and after a visit to the foundry and having castings made, the costs are just mind boggling. A locomotive like you suggest being made by commercial builders working on the parts would probably cost way in excess of $500,000. Being built in house or in your own shop could cut down on the cost, but it would still be considerable.
The next thing would be time. #9 has taken much more time than estimated. I see #11 as a 10+ year project. Would #53 be worked on after #11 is done?
These are all things to think about when entering a project like this.
Bernie
