My power is gone...

So I've got a strange little problem. I run a Grubee Skyhawk 48cc with a CNS carb and NGK B5HS plug gapped at .025, with a fuel/oil mix of 20:1. With my 48t sprocket I can usually get around 28mph on flat ground and climb the huge hill in front of my house at about 17mph (huge hill!). But lately, I'm doing good to get up to 20mph on the flats and can't climb the hill at all without pedaling. When I'm running WOT on flat ground, the engine sounds like it isn't getting up to full RPMs. I have no exhaust leak, but when I took the end off of the muffler there was a lot of oil laying in the bottom of it. I've taken the carb apart and cleaned it and the air filter, changed the fuel filter and out of desperation, tightened my head bolts. I found that they weren't loose to begin with, but I was afraid maybe the head gasket was leaking, not the case as far as I can tell. The plug is a little fowled, but I've had worse and it ran better. I was going to try a new plug anyway, but I thought I would ask around here first and see what you guys have to say.

I noticed a few other threads about engines with low power, but they seem to be different than my situation. Any help at all would be greatly appreciated.

So you think my gas mix has something to do with it? I was wondering about that as well, that's why I didn't just throw a new plug in and go.

So I figured it out. I don't have any idea how I missed it, but my head gasket is leaking pretty badly. I saw the oil all over the cooling fins, but I had a gas line leak recently and I figured the oil was left over from that. Anyway, it's not just that, it looks like the cylinder head is warped. You can actually see it, I've included some pics. In the second one that you can see light from my flashlight coming through between the gasket and the head. I know it's blurry, but it was a hard pic to take.

So Now I have a few questions. For one, what would cause this to happen? could have always been warped, but the gasket held it back from leaking up until recently? Did I over heat the engine? Are these cheap china engines always like that? Also, I went ahead and ordered a new head and gasket, but I couldn't find a head that was rounded and black like the original one. Anyone know where to get one like that?

Quick update.

I took the head off and it is actually not warped. The casting has a curve to it that made it look warped, but the surface that is in contact with the head gasket is machined flat. So what I have here is a run of the mill blown head gasket.

Okay, quick update. I'm starting to feel like I'm talking to myself in this thread, lol. But this is important info. It turns out that the head gasket was not the only problem. My airfilter, despite my cleaning efforts was soaked with oil and thus not letting enough air through, this was contributing to my lack of power.

So if you have a CNS carb, CLEAN THE AIR FILTER REGULARLY with dish soap and water. apparently flushing it out with solvent does not remove the oil like I thought it would.