Changed brush roller and belt after vacuum pick-up became poor, but now belt just slips off motor spindle after a few seconds of use? Looks like there may be something missing on upper casing to prevent belt running off?
Looking at the explosion there is no belt cover on the motor spindle, but there is a belt guide cover over the brush roll.
Assuming you have fitted the correct motor and belt the only other thing I can think of is the chassis may be twisted which causing the belt to run off.