There are two spark plugs that fit these engines. The plugs are interchangeable in the engines but not in the shielded caps. The treads and reach are the same on both spark plugs.

The smaller exterior spark plug has a tapered seat. NGK BPMR7F

The larger exterior size spark plug has a gasketed seat. NGK BPMR7A

The NGK BPMR7A spark plug should be in stock at most any auto parts store that carries NGK. The BPMR7F is usually not stocked locally but they can get it for you - often by the next day