The fan on there is a Corsair SP120 High Performance as a point of reference.
Should you choose another option, you really stick with a fan that at least meets same cfm AND static pressure specs. Yep - not just any ol fan is a good idea. Rads need static pressure (SP part of the model designation on your current fan) to push or pull air through against resistance. IIRC specs are 3.2bars h2o and 58cfm.
Corsair SP series are quite good rad fans and around 13-15usd