Kevin,
I think to answer your question, the answer is that there is no one company that could build all of the locomotive. That being said, I believe that a coopertive effort could be made by several companies for components and that there may be firms out there that could do final assembly (Strasburg RR). I beleive that they are currently building a replica locomotive fo a V & T engine. Monticello RR Museum just had a new boiler built for a rather large 2-8-0 and I have seen it with my own eyes. It was made with the new techiques in mind so I am told and it looked great. I was told that a firm in St. Louis manufactured it.
It will be interesting to see if a brand new locomotive could be built for Railstar.
Just a few of my thoughts.
JD Johnson, Morehead and North Fork Railroad Historian