Reverse Engineering Fighter Aircrafts – How successful and currently is it viable??

Would appreciate information on this. Everybody is aware of the Chinese and their success at reverse engineering old soviet equipment including the Mig-21 and other military hardware. The Israelis did it with their Lavi, I believe, based on the F-16, but stopped at the prototype stage, why??? was it unviable??? Now the Chinese are developing that into their J-10?

Do the Americans and Russians indulge in this too?? or do such things stay at critical new avionics, EW, and other cutting edge technology rather than the whole aircraft.

Would it be possible to reverse engineer a current (fourth to fifth generation) fighter aircraft from scratch in a reasonable amount of time.
a) What kind of military industrial infrastructure is required.
b) Would there be violation of intellectual property rights in such cases.
c) How successful would such a reverse engineered aircraft be, would it be able to perform to the standards of the original.
d) Would the economic investment in such reverse engineering be worthwhile rather than buying comparable planes off the shelf?