Here's a link to an unclassified video which gives a description of the F-35 capabilities vs a 4th generation fighter.
Build a Mig-21 or F-5 with more efficient powerplant and integrate with modern weaponry.
Where is the "more efficient powerplant" coming from? I would expect the U.S. Government and NATO governments would restrict engine sales to Turkey. So, the engines would have to come from Russia or possibly China. Same thing for weapons and avionics. IMHO, Russian and Chinese weaponry are inferior to F-35's.
Development time, at a minimum, would be 10 years. Then EMD and LRIP would be 5 years. F-35s are being delivered now.
Bottom line...Turkey is giving up the F-35 for what?
It is politics above all. I think Turkey’s idea is it won’t be manipulated by either party and free to choose itself. In previous discussions I wrote the perception of the situation from inside Turkey.