Diagnosing power loss

Hi again everyone,
I have a new issue I could use some help with. My 1996 ja22w (k6a) is loosing power after a few minutes of driving. Everything runs smoothly when it is first started, but after less than 5 minutes of running, I only have about 50 percent of my power, the turbo becomes quieter, the valves (or exhaust leak) become more audible. From my research, I'm thinking maybe o2 sensor or coil, but I have a very hard time finding parts so I'm hoping someone can help me with a sound diagnosis before I break out the wallet, or if anyone has experience with wiring a universal o2 sensor or universal coil? 
open to any advice!

Have you checked the condition of your exhaust? If it's blocked in one of the catalytic converter then that will reduce power and make any leaking more obvious.

Very hard to diagnose remotely. Check the valve clearances. Do a compression test. Check for boost leaks.

I'll take a look at it while it's running. I've heard blocked cats get noticeably overheated.

Thank you for the response! Are you willing to give me some details about how one goes about checking valve clearances?

Best to watch a few videos or find a good handbook that talks you through the process. It's not difficult, but it is a bit more involved than regular servicing.
In simple terms you remove the valve cover, rotate the engine to put each piston at top dead centre then measure the clearance between the cam lobes and the valve buckets.

Update;
I ruled out the ignition coil. It especially struggles over 3k rpm. Idle is a bit shakey sometimes. If I'm taking a hill (usually over 3k rpm) it struggles and gives a check engine light. As soon as I let off the throttle ( within half of a second) the check engine light clears. I'm thinking fuel delivery issue? Does anyone know spec for running fuel pressure? Or fuel flow? I'm not well versed with fuel issues so any diagnostic advice is welcome!