Let's face it: used diesels are nearly a dime a dozen here. Except for museum purposes (such as the streetcars exported to South America and then brought back to the USA decades later for use in museums) there would be no real advantage to bringing a locomotive back here. For the prices that it is possible to buy a used diesel, it is hard to imagine it making economic sense to ship one all the way back here.
The other problem is gauges. Many USA made diesels went to Africa ( some meter gauge, lots of 3 ft 6 in gauge) or a few in India (five foot 6 inch gauge) or South America (meter gauge, 1.6 meter guage, five foot 6 inch gauge, and some standard gauge). Therefore, there are only a few select places where an export unit would be operable on track in the USA if it ever came back. Some meter gauge and 1.6 meter gauge units in Brazil are "off the shelf" USA units with different gauge trucks, but the conversion would still be an expense.
Loading gauge is also a problem. GE's "Little Joe's" fit the Milwaukee Road, Chicago South Shore and South Bend, and the 1.6 meter gauge lines around São Paulo. However, I doubt they would be able to run one of those on the Northeast Corridor. The line side clearance is too narrow. I think this goes for some of the other stock used on the 1.6 meter and five foot 6 inch gauge lines.