I'd do a compression test first . If you have a burnt valve , it's very likely it wouldn't run very well , if at all .
It only backfires on deceleration , which indicates an air leak in the intake .
The stock carb for that scooter looks like it may have an air cut valve . If so and that valve diaphragm is defective , it would allow decell backfires .
As a test , try turning the pilot screw out a few turns , if it has such .