I wouldn't feel comfortable with 31 min at LAX. You're leaving JFK at the beginning of afternoon/evening crazy-time and if there's any kind of delay at LAX getting to your gate, which there frequently is as many LAX gates are tow-in, then you have very little margin of error. As stated above, if you have to switch terminals, add another ten minutes of walking time. I'd stick to what you have and get some In-'N-Out Burger or have some cocktails at Encounters.