Here is a video, Might help a little but not the plug:
Husqvarna Lawn Tractors: Maintenance - YouTube

The following is a photo of the engine, the plug will be at the end of the cylinder, or end of Plug wire:
http://www.husqvarna.com/us/products...tors/yth21k46/
Looks like it would be under the Metal cover with "OHV".
Good Luck