My 1976 B&S had a couple of stripped cowl screws when I bought it. My cure was to drill, tap, and replace them all with one-size-larger (so I won't have to mess about with two sizes of spanner on that engine). From memory they start out at 1/4" UNC and I replaced them with 5/16 UNC. The original size is fine functionally, but it can't deal with the clumsiness of the people who sometimes work on mowers.
Because the castings are usually aluminium, chances are it is the casting that is stripped rather than the screws. New screws will not help much.
