Manufacturers normally leave some margin for varying loads and power supply voltages as well as a factor for expected life span. Most motors also have some headroom for overload included in their design (a multiplier known as service factor).
You have nothing really to loose by trying it if in all other important respects it matches--it might run slightly warmer than the 1/3 hp model, but should otherwise be tolerable since the motor probably will not be fully loaded anyway.
You can always check the current draw of the motor (with an ammeter) to infer the actual load while it is operating. If the current draw (amps) is less than or equal to the nameplate rating of the motor (at its designated voltage) then you are in the safe operating range. If it is over, then you may have to reduce the duty cycle [on time] slightly to prevent it from exceeding the maximum temperature rise permitted (again indicated on the plate). Another basic rule of thumb is if you can keep the maximum temperature rise at or less than the rating on the motor (typically 40deg C. above ambient), you will be okay.