25-1 is still too much oil but it would not account for that much loss of power.

Where did you get the header you are using? What type is it?

Have you tested the engine with the muffler that came with it?

I've seen a couple engines that had not-so-good piston rings in them. They perked right up when the rings were replaced, but it's impossible to tell if that is the problem from here.

TF