Bob,
You're right. As a "rich guy wants a toy locomotive" goes the Montezuma would be a sweet way to go. Powerful enough to tackel a mountain with a parlour car, but small enough to transport on a single truck.
If profit is not a concern then it's a great idea!
Actually on the Montezuma the cylinders are cast seperate from the saddle, which on the real thing was a sheetmetal fabrication. This means that a single cylinder pattern can be used to cast the left and right side.
You're right that she is an amazingly simple engine. I'd have a kick designing a replica because of the simplicity. I've worked on live steamers more complex.
Curtis F.