Those bikes were not very fast. They were geared fairly low to allow the little 47cc engine to move them but the top end mph just wasn't there. I remember riding one of my Sachs powered SSSCAT's and thinking...that's it? Second gear was revving pretty high and I know I was going less than 20 mph.
Could check to see if your brakes are rubbing.
Try elevating bike on a stand and allow the back wheel to rotate freely while bike is running. Might find something holding it up, or not.
Mike