Drew
In Regard to the reccomended spark plugs for Atco mowers. There where two types of Villiers Engines used on Atco Mowers, both with different Cylinder heads. A 14 mm and an 18 mm. The 14 mm took a Champion L90 spark plug and the 18 mm took a Champion D16 plug.
Hope this is of some assistance. Let me know how you go

Regards Ben.