Scooter has trouble getting to high speeds

So I recentley bought a second hand scooter a; Solifer e4-bt49qt-12, when I bought it, it was running smooth for a few weeks doing 55 kmh or 34 mph (with a cdi box restrictor), but about two weeks ago the drive belt snapped, well no problem there I went ahead and bought a new one, I immidietaly noticed that the scooter wasnt able to get up to the same speed that it used to be able to get to, as now it is only doing 24-27 mph or 40-45kmh. I decided to take out the cdi box to see if it would make a difference, well it did and it didn't, down hill I am able to get to a good 50 mph but on a straight it is still doing 24-27, I have checked to make sure that the belt is the right size and it was, I have no idea what the problem could be but I really need to find a fix as it is my only mode of transport and I have to leave really early to make it in time to school and other activities. Any ideas will be appriciated.