I would be careful about using sliders/rollers that do what Dr. Pulley shows in their video above. What the video does not show is that it eats into the safety margin that keeps the belt from flying off the pulleys.
Here is a video presumably with factory rollers. Notice how close to the maximum diameter of both the front and rear pulleys the belt gets. If the Dr. Pulley system can squeeze the front faces together more as they show to get higher speed, the belt would be close to exceeding the diameter of the front pulleys on the bike in this video. The same is true if the front faces can move farther apart giving a lower, low end. The belt on the rear pulley then would get nearer to the maximum diameter of the pulley.
The Dr. Pulley system can work fine so long as the pulleys are large enough in diameter that the belt cannot come off. The problem for Dr. Pulley is that they cannot know just how close the scooter manufacturers safety margin is but you can check it just like in the video below.