Lots of great suggestions.

How long does it take to start loosing power?

It could be a worn out piston/liner as suggested, but be sure to check over the fuel system very carefully first. 8months is an awful short time to wear out an engine unless it's been sucking in a lot of dust or been planted and not cleaned out fully. (unless you're the type who flys 3-4 flights every day). If the engine doesn't die, just looses power for no apparent reason, and no matter how rich you set it, and you see nothing wrong with the fuel delivery, this becomes more likely. Fwiw, a piston/liner swap isn't hard to do, and the parts cost $35 from tower right now. If you really suspect the engine is worn out, try running the engine on a stand first to make sure, or at least hook up an external tank on the airplane with all new tubing, and run it that way (I don't suggest flying that way, just run the engine a bit).

Quick question, when you replaced the plug, did you look at the old one? Was the plug element still in the plug or was it missing? Often times the plug element will blow out of the plug and just go harmlessly out the exhaust port, but sometimes it will get stuck, and can score up the piston/liner.

8months, flying a few flights most weekends is about the right amount of time to start seeing things like the loose or cracked backplate (those darn plastic backplates), loose head or carb, or such. I've also seen holes appear in fuel tanks from rubbing against screws inside, or fuel lines developing holes due to vibrating against the metal fuel tank tubing or other parts of the plane or tank. 8mo is a bit short for the fuel tank stopper to have worked loose, but check it anyway, they sometimes loosen up over time. Also check the muffler.

A good thing to look for is dark colored oil or gunk that has started to appear around the engine compartment that wasn't there before. That would point to loose engine parts.

On the list of silly things to check first, make sure the clunk isn't stuck at the front of the tank.