You can credit card as many ready-to-run 9L+ V8 engines as you want[1], built to modern tolerances using high quality tooling, tested on a dyno and equipped with a warranty. Maybe different mounts and/or couplers are needed, but that's a very finite problem.
I suspect one actually difficult problem would be finding a refiner that can make the special fuel used in the unique engines of the SR-71. Then again, the US has a great many agile refiners at its disposal.
The SR-71 is a rather awkward thing to operate. Fuel leaks out of the tanks until atmospheric friction heats the aircraft enough to make them seal. It's a big mess, and the cohort of people that were actually capable of making these things work is aged out of the workforce. Assuming any of this real, NASA must have something truly extraordinary in mind to consider this.