The CD50 disc spindle motor is a brushless motor with its own control circuit. The coils or Hall sensors may be faulty, but the first thing I should check is whether the 'disc clamped' switch closes. It should engage once the tray is completely closed and the disc is in position, and should give a positive 5V on pin 19 of 8IC310. The switch may be dirty as a result of the same 'fluff' you pulled out of the motor mechanism.
Does the carrier servo move at all or does it stay in the home position? The disc detection mechanism can also be at fault, it's a little light gate (LED/photodiode assembly) that detects whether a disc is loaded. If the optics are dirty, it will not engage either.
Both are probably part of a safety interlock circuit that disables the laser if there's nothing in the player. Give the player a good cleaning, that's always a good place to start. Be careful with the laser lens -- any grease or dirt on there (from your fingers) can interfere with its operation.