In practice it will probably be fine, brushed DC motors are typically extremely tolerant to over (and under) voltage, most 120V brushed motors will run on 6V or less, and for low duty cycles 200% of rated voltage shouldn't be a problem. There are several limiting factors, one is the bearings and other mechanical parts, if you spin the motor too fast it can literally come apart. The next is thermal, the more power you push through it the warmer the windings will get and at some point they will burn up. The remaining and usually biggest limitation is the brushes, if you push too much current through those they can fail quickly, even catestrophically or just overheat.