What was found
(CAN) ACFT WAS ON THE GROUND, PILOTS WERE UNABLE TO START THE ENGINE, HUNG START. SO THEY SHUTDOWN, AND OBSERVED SMOKE FROM THE BACK OF THE ENGINE. THEY EXITED THE ACFT, NO PASSENGERS ON BOARD, REPOSITIONING FLIGHT, WITH FIRE EXTINGUISHERS IN HAND, BUT THE SMOKE DISSIPATED QUICKLY. MAINTENANCE REPLACED THE S/G AND DISCOVERED A SHEARED OFF COOLING FAN, NO FURTHER DEFECTS OR PERFORMANCE ISSUES.