I have never owned a CatEye but what is your front sprocket and rear sprocket amount?
Basically you would either enlarge the front sprocket or reduce the rear wheel sprocket to gain top end. As far as I remember one tooth on the motor sprocket is equal to 10 on the rear. Remember when you change the rear you will definitely need to change your chain or reduce the links.
It has a jack shaft with double barrel gear and 2 chains, one from the trans to the barrel and one from the barrel to the rear sprocket 8mm T8F chain. The shaft is a bending point for the rear arms and shock.