If your gas scooter originally had 14-inch wheels, it's not advisable to replace them with 13-inch wheels. The size of the wheels on a scooter is crucial for its stability, performance, and safety. Using smaller wheels can affect the scooter's handling, balance, and ground clearance. It may also result in the scooter's frame being closer to the ground, which can lead to scraping or bottoming out on obstacles. If you cannot find the exact replacement wheels for your scooter, I recommend reaching out to the manufacturer or a reputable scooter parts supplier to inquire about availability or alternative options that are compatible with your scooter's make and model. Installing wheels that are not the correct size can compromise your scooter's safety and performance, so it's essential to find a suitable replacement.