JAE is right, have a look at the clips on the side of the fan
that hold it to your heatsink. That looks like the sole method of securing the fan to the heatsink, so a standard case fan will require some jimmy rigging.
Why not just get a replacement?
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod