omega1,
I doubt they even have the tools to build an SR-71. They were built back in the 60s. The fuel tanks of the SR-71 were at one time lined with gold to
help heat distribution, but they started corroding.

As for new powerplants, I doubt that as well. The J-58 with its inlet spike is pretty much the
pinnacle of turbojet technology. You can't really make it run any faster or better than it already is. Combustion occurs at subsonic velocities and
more than 50% of "thrust" is suction caused by the spike. Very clever, but you're not going to get a much better aircraft that way.
IMO, the SR-71s were being used as testbeds for something.