is it safe to run a 300 watt 220v motor (european) on a 20 amp breaker (american)?
The 20 A breaker is the maximum the circuit can provide. Your motor is rated 300 W so it should use less than 2A and be fine. The concerns are frequency - 50 vs. 60 hz can affect the speed, and by extension safety and efficiency, of the motor, depending on design. In addition any used or "unknown" device, particularly if not rated /tested by a reputable source, might turn out to be a safety hazard even under normal use due to issues with insulation, grounding, or other problems.