What spark plug does the Craftsman 917.275702 use, and how often should it be replaced?

The tractor uses a Champion QC12YC spark plug with a gap setting of .040 inches. Replace the spark plug at the beginning of each mowing season or after every 100 hours of operation, whichever occurs first. A new spark plug helps ensure the proper air-fuel mixture and helps the engine run better and last longer. (Page 6, Page 20)