You have to get rid of the old gas. If you just top it up with new, and seafoam, it will run badly for a tankful. Then fill it again, more seafoam (like an ounce or 2) and it should get better. After a few hundred miles things should be back up to par.
Be very careful with those old dried out hard tires. They can kill you, and ruin your scooter, when they pop at speed and you go flying.