: "Spion Kop" is the spelling that's most common in English. To confirm this, I did a Google search, restricted to pages in English. I got: 

 :* "Spion Kop" - 6,330  
 :* "Spioenkop" - 1,490  
 :* "Spioen Kop" - 37 (just to check to see if this gets any use) 

 : Since policy on the English Wikipedia is to use the English version of names, please leave it where it is. Thank you.