Spark plug is fitted into the cylinder head - somewhere!
Cylinder head is at the top of the engine that has a vertical or nearly-vertical cylinder or with a horizontal cylinder is usually the forward-most part.
Spark plug has a thick wire connected to it so if you can see a thick wire try following it...
If you are thinking of replacing your bike's spark plug, take great care when refitting. The threads are fine and cross-threading is easy to do so always fit the new plug and screw in at least two full turns by hand and take great care not to overtighten it.
Most new spark plugs are fitted with a collapsible washer to ensure a gas-tight seal. The washer can be felt collapsing in the last approximately 60 - 90 degrees of turning and equally it can be easily felt when it has finished collapsing. From that point it needs only a few pounds of additional pressure on a 6 inch wrench to finish the job.
When refitting a plug with a used washer, once contact with the cylinder head is made, it only needs that small amount of wrench pressure for it to be tight enough.
on Dec 16, 2020