When you say the electrical system is perfect, are you including timing? If everything is working, but that big blue spark comes at the wrong time, it'll really hurt performance.
Make sure the air filter is clean. Probably so, since the plug chop came out OK, but ckeck anyway.
The exhaust system may be clogged, which doesn't let the engine breathe. If the exhaust can't get out, the air/fuel can't get in. We're talking muffler, baffle, pipe, exhaust port. All get clogged after a while.
Does your engine have a reed valve? I don't think so, but if it does, that could be faulty.