Speedfight 2 bogging and without power

I have a Peugeot Speedfight 2. It got a new piston that is similar to the original (but it has lubrication windows), and the carburetor was cleaned. When everything was assembled, it seemed to be working fine, but after 2 minutes, it started backfiring and bogging down. It gradually lost more power, backfired, and bogged down until it only revved a lot and didn't move. It has compression, and the spark plug came out oily, along with the head gasket (which is unusual). It doesn't have excess oil, and there shouldn't be oil there. The spark plug was replaced, and it stopped backfiring, but it still lacks power, even though it no longer fouls the spark plug. Any solutions?