Calling on Shade Tree Mechanics

I have a 1992 toyota camry station wagon (320,000 miles on the clock) with a 3.0 liter engine.
It has begun to run rough. So I pulled the front plugs and found a carboned up spark plug. I didn’t pull the back plugs as one has to remove the intake plenum. Any way replacing the plug did solve the problem temporarily.
I am guessing that I have either a partially or fully plugged fuel injector or a bad valve oil seal. Could anything else be the cause? Any idea if one can determine the exact cause?
My plan of attack is to dump fuel injector cleaner into my gas tank at every fueling occasion. If the plug fouls again I would spend the money to have that fuel injector cleaned or replaced. It shouldn’t cost much as it is right there on top of the engine.
I don’t think this vehicle is worth fixing the oil seal as the cylinder head would have to be removed. At $100/hour labor cost; that would result in a big buck repair on an ancient vehicle.
Thanks for any input.

So exactly what do you mean by rough? Is it missing, the car shaking, low power? What I would do is go on and put in your car and problem you are having. Usually they will have video’s helping you resolve the problem.