Could be compression, burned valve, misadjusted valve, valve sticking/ bent
Could be ignition
Could be something AKA leak causing "lean" in one cylinder. Pay particular attention to "how even" plugs look
Here is what you do
Beg borrow or steal a compression gauge. Google "leak down tester" you can actually make one if you are handy, and use your compression gauge for "parts" for the leak down tester. Lacking that, buy or make an air fitting.......or use the parts of your compression gauge........so you can inject air into each cylinder and listen at exhaust, carburetor, etc for leaks
Next do a power/ load test. This will show what is weak and maybe if there's an ignition problem
