If the piston was installed backwards during that rebuild, the loss of power would be described just as you have. Later, when the ring end catches on a transfer port and munches all those new parts, you will know for sure.
Pop off the cylinder head and make sure the arrow atop the piston points toward the exhaust side of the motor. If the arrow points back toward the carburetor, take the top end apart, replace the gaskets, and put the piston in with the arrow facing the exhaust.
If that's not the case, you might have a rat living in your airbox, or a wasp nest in your silencer. Make sure both the intake and exhaust are obstruction free just to be sure.
Good luck,
Jon