Oil it has plenty, since the recommended mix is 1:40.

Is the thermal grade of the new spark plug similar to the old plug?
Might it be too hot for that engine?

Besides checking the timing, as recommended by w8ye, I suggest inspecting the exhaust port, and also the muffler, for any obstructing buildup.
For that, you will need to disconnect the muffler.
Excess or less than ideal type of oil can lead to those buildups, which restrain the proper ventilation of the hot exhaust gases.

Regards!