The 5HP Briggs uses the same gas cap as the 2-4 HP regardless of what that Amazon ad states. You want Briggs part # 298425 which has been superseded to # 493982S. (Do a search on eBay using that #) I would recommend staying with the actual Briggs OEM brand, not the aftermarket ones. In the past I have purchased some of the latter and about 1/2 of them leaked.
Michael