1992 Toyota Celica Engine misfiring after warmup

I have a '92 Celica All-Trac turbo, 2.0L engine AWD. I've had it for about 7 years, it has seen quite a bit of modifying over the years, but currently is back to stock formation and has been for about 3-4 years.

I take good care of the car, always changing the oil routinely myself and have never had any mechanical engine problems. A few months ago the check engine light came on after starting it up and it began misfiring once the engine was warmed up. The car had an extremely difficult time making it up a moderate hill.

I checked the spark plugs and found the number one plug had more white buildup on the electrode so I performed a compression test. All of the cylinders are much higher than minimal tolerance, much higher than stock actually. This likely due to the amount of time the car ran extremely rich with a larger turbo and other modifications. But all of the cylinders are almost exactly even as far as psi measurements.

I wanted to do a leak down test to see if there is a valve problem but I don't have access to a tester. I am really at a loss for what can be going on. I was thinking there is a possible injector problem since the car runs fairly smoothly when cold.

I should note that the CEL code is for an engine misfire and knock sensor. I have had issues with the knock sensor in the past. Could the misfire be due to the high carbon buildup on top of the pistons?

This is not my daily driver so it is not urgent, but the car has been down for quite some time now and I would like to get it back to driving condition soon.