I had this exact same problem with mine. One option was getting bigger screws, that won't work because the holes in the plate itself are only so big. Another option was to use one of the lower holes in the plate, bit then things aren't symmetrical and you have a useless hole where the screw was stripped out.

What I ended up doing was going to Home Depot and picking up some Hollow Wall Anchors, Medium Duty. The manufacturer is Crown Bolt. I got this size: #6-32 x 1". You just drill out the stripped hole large enough to put the wall anchor through and then crank it down. I put a small washer on the head of the screw because the screw itself is a little small for the liscence plate. The hardware is stainless so you don't have to worry about rust and what-not. The solution worked so well on the stripped out side that I decided to do it to the other hole as well.

Let me know if you have any questions.