I'm sure some of the materials on the F-35 would be very difficult for other countries to make at scale, figuring out how to fabricate parts out of those unusual materials at tolerances required for low-observability would be really tricky. I'm sure there are machinists anywhere who could figure it out, but at that point we're talking about bespoke part production using 98th+ percentile operators. Although skilled labor is cheap is China and Russia, it's pretty untenable to use it to produce a knock-off that'll eventually hold an inferior Chinese/Russian engine.