There is more heavy manufacturing capability left in the US than you might think. All of the capabilities that you mentioned are still out there. Where you may run into a brick wall would be in trying to duplicate a one piece cast steel bed frame and cylinders. I doubt that we, or anyone world wide still has the pattern making and foundry ability to do that. And if steamers were still being commercially built today, the ones with one-piece frames would have them built up out of weldments anyway. But that is out of the realm of this discussion.
Also out of the realm of practicality are reproductions of the latest and largest steam engines. Not only would they be vastly more expensive with their roller bearings, box pox driver castings, one piece frames, and huge boilers, few people can find anywhere to run the big engines that we already have. Why build more?
But when you get into the mid range, and smaller engines, the ones with spoked drivers, built-up frames, friction bearings, and manageable boilers, and certainly the ones capable of being moved by truck, the practical problems of building one become a no-brainer.
The old builders had a basic system to make a ballpark cost estimate for a new engine of usual design, by the pound. Engines like the #315, the K classes, and say up to the USRA designs, were all very similar beasts, and built of the same basic metals, just larger, or smaller in size. If you looked up the prices of basic, say 1930 and earlier engines, and adjusted for inflation, and for higher productivity of the more modern tools used to build the later ones, you would find that the cost per pound to build would be about the same throughout.
We can use the same technique today. Just off the top of my head, using the input of this discussion and my own experience, I would say that we could have some faith in using $40,000 per ton of finished engine and tender (empty). This doesn’t go for the exotic stuff, PRR T-1’s, or anything considered “super power”, but for basic steamers, certainly up to 100 tons, my bet is that it wouldn’t be far off.