You can run a damper on either the blower inlet or outlet and it's not likely to make much difference to either the temperature rise or the power consumed, at the sort of pressure differential you can expect from the fan.
Broadly speaking, throttling the flow to half its unthrottled value will halve the power consumed. The "volume to power" ratio is constant.
With 425 watts absorbed, the fan probably represents a fair chunk of the output of the generator it is intended to cool. It may be worth considering a speed controller instead.
The "fan laws" give the relationship between flow, pressure, absorbed power and fan speed for any given fan.
Flow varies with the fan speed.
Pressure varies with the square of the fan speed.
Power varies with the cube of the fan speed.
If your fan uses 425 watts at maximum flow, it will use 212 watts when throttled to half flow.
It would only use 53 watts if the speed (and therefore flow) were halved.
The cube relationship between speed and power means that very little torque is needed at low speeds, so a very cheap, simple speed controller can be used.
Obviously, speed control is an added complication and goes against the KISS philosophy. The simple fan speed controllers may also play merry hell with power quality from a small generator (I don't know, it would need checking), but if output is tight, it could be worthwhile.