The engine does not relate to stealth, unless you are referring to nozzles.
The engine is crucial for stealth, along with the nozzles.
For one, you need augmentors in the engine, these are radar blockers placed inside the exhaust to block EM waves.
This is from an article from Aviation week and I don't have a link to it.
Pratt points out that the F119 and F135 are the only production engines with stealthy augmentors. Their design eliminates conventional spray bars and flame holders and integrates multi-zone reheat fuel injection into curved vanes that block the line-of-sight to the turbine.
These augmentors are clearly visible if you look inside the exhaust of the engine. And this cannot be "installed" on existing engines, you are going to have to design one from scratch. You can look one up for the F-22.
Then you are going to need the latest IR reduction techniques, not including the widely known ceramic coating on the nozzles. Not to mention any number of unknown techniques being used today. IR reduction is another aspect that has to be designed into the engine.
As for the nozzle design in particular, it only affects RCS at certain altitudes and angles which can be taken care of with active cancellation techniques. I am not particularly bothered by the shape of the nozzles, but I am very interested in the path the FC-31 will take with regards to the engine. With the RD-93, the FC-31 is as stealthy as a J-10B.
Engine thrust does matter in delivering the kind of weight-to-thrust ratio to allow super-cruise, which is considered one of the key 5th-gen specifications.
If you ditch super-cruise, but retain many of the other 5th-gen aspects, like stealth (at least frontal and side), AESA, other advanced sensors, battle-field network and information share capabilities etc, then you can still make a very attractive and capable aircraft by-passing the bottle neck in the engine department.
The main advantage of super-cruise is in operational range. Cruising in supersonic speed dramatically decreases drag, and allowing much better fuel efficiency. Airplanes which cannot do super-cruise require after-burners to reach supersonic speeds, which burns off fuel very quickly, and they cannot maintain that speed with normal engine power.
These are all existing qualities of current European 4th generation jets like the Rafale, EF and Gripen C/D/E.
Rafale and EF are known to supercruise between mach 1.2 to mach 1.5, both with and without tanks. Both aircraft have high thrust engines, and the avionics you mentioned.
But for nations like Pakistan, and if the FC-31 are mainly to be used in air defence roles, then operational range isn't the highest of priorities. For a cheaper price and quicker delivery date (no new engines to hold things up), you still get most of the other more important 5th-gen fighter characteristics. So it may very well be a worthwhile trade-off.
Offense, defense, you are going to need range for everything. And without 5th gen engine, which is as important as a 5th gen airframe, you are not going to get anywhere far with the FC-31.
And you can't trade off anything on a 5th gen jet. There is no such thing as taking half measures when you talk about 5th gen. That's why the Americans are skeptical about anybody else getting a 5th gen jet flying before the F-35. And unlike previous reports, the F-35 is also an all-aspect stealth aircraft. No half measures there.
