Personally, I would strip down the PSU and find out what fan is in it and get the specs from that. Yes, it means an extra step but it is the safest option. If you are lucky, you may be able to see the label on the back of the fan without opening it up too far. Even just the model number would be enough to track down the specs.
Whilst fans may seem the same (diameter, thickness, voltage etc.) they can have wildly varying properties. Regardless of any other spec, airflow is the key as the fan will more than likely have been "chosen" primarily on how much hot air it can shift. Replace it with a fan that moves less air and you run the risk of overheating.