There are many things that can cause lack of power. From bad gasoline to a bad spark plug to a weak spark, to low compression. I would take the spark plug out and check the engine compression. If is above 120 psi. it is ok. check exhaust port for carbon build up, check for blocked muffler. Check main jet for partial blockage. Check for rust deposits in the gas tank. You have lots of work to do. You need to work systematically.