I think it would fill a gap in the market for definite, there are many classes of loco out there just crying out to be made for the game. For me personally, I've not a great amount of interest in the class of locomotive, though I do fully appreciate their design and the look of them is unique. Of course, we are all entitled to our opinions.
Sound recordings I would imagine would be the limiting factor on the project such as this..... I presume (with the photo being taken with the loco outside the NRM Shildon?) that the loco no longer runs due to it being a part the the collection (Yes, before anybody says it, Flying Scotsman is an exception, as is/was Green Arrow). Unless there are locos that sound like it that an author could "borrow" and adapt to sound like a C1? As I said I'm not familiar with the class of loco.
For me personally (slightly hi-jacking the thread here) I would love to see one of these in the game for the very same reason you suggest, S (in the sense of it being unique, or in my opinion anyway)......
(Image taken from "
http://www.semaphoresandsteam.com/", full credit to the author(s).