They can't make one that generates reasonable thrust and lasts for more than 12-20 hours.
Even with the precise engineering documents they have stolen.
The Russkies had the same problem. Unable to make precision bearings that would endure extreme duty. In the book, “MIG PILOT”, subtitled, “The Final Escape of Lt. Belenko,” after he had defected to the west via Japan, as he was being debriefed by our people, we were amazed to learn of the MIG-25’s speed limitations, having tracked one at some ridiculous speed, above Mach-1? I forget.
Anyhow, Belenko stated that the flight we tracked had been staged as a snow job for our people. He said that after that extended burst of speed, the engines were shot and had to be replaced. He said that Russia couldn’t make good bearings.
That was a very revealing book.