"What would be the economics of building a new fleet of C47/DC3s from scratch for GA purposes, including nostalgia displays and enthusiasts' flights? Presumably the blueprints are still available. Or would the cost of manufacturing new recip engines be too much a problem?"
THE problem is not rebuilding a DC-3 from scratch - it's the cost of certification - you wouldn't be able to use a 1935 Certificate - it would have to be done to 2019 standards I believe if you want to carry passengers. You might get an Experimental Certificate in the USA but that doesn't allow you to ply for hire.........