atx psu power ratings are notoriously overspecced - 450w is probably closer to 300w real world and along with the fact that you have multiple voltage rails the actual output on the 12v rail is prob around 10A - so possibly doable but I would be inclined not to if as you say there is any chance the motor will stall - but bear in mind IANAEE
atx psu power ratings are notoriously overspecced - 450w is probably closer to 300w real world and along with the fact that you have multiple voltage rails the actual output on the 12v rail is prob around 10A - so possibly doable but I would be inclined not to if as you say there is any chance the motor will stall - but bear in mind IANAEE