4.ENGINE INLET MODIFICATION:
The pak fa engine air intake and pod does not seem to form an "S" is recommended for stealth aircraft. The front of the engine is one of the main reflectors in the front and the Russians should use other means to block air intakes.
They should use radar blockers or modify the engine inlet by adding a coat in the inlet duct with radar absorbent material, because a lot of radar energy bounces off the duct wall several times on the way in and out again. High-level stealth, though, means physically blocking the line of sight with a "serpentine" duct (which is done on the F-22, JSF and Typhoon). But that can take up a lot of space, particularly with big engines, and isn't practical for a stealth retrofit or on some new designs.
The Super Hornet, for example, has short inlet ducts so line-of-sight blockage by curvature isn't practical. The solution was to install a blocker in the inlet duct - looking down the duct, you see what looks like a compressor face, but isn't. It's a fixed composite structure, RAM-coated.
ANOTHER INTERESTING CONCEPT OF ENGINE INLET MODIFICATION
It uses flexible vanes with a rotating ring at the rear end: in the "stealth regime" it provides extensive blockage, but it clears the airflow when it doesn't matter or you need full speed or power. One challenge would be the structural design, because the last thing you want is a piece of RAM flaking off the moving surface and FODing out your motor.