OK, first off the ad is for a Briggs and Stratton "World Formula" engine. That is not even close to a "flathead." It is a purpose built class racing engine for go karts, and it is OHV.
The add itself lists several components that come with their kit, but most of the components don't even come on a BS WF engine, so I don't know what it is they are selling, other than basic motorcycle jetting kits for three times their value.
So no, this is not it, not by a long shot. A World Formula engine is about $900 USD and up.