The radiator type will be the most efficient but only if kept spotlessly clean and unclogged from mud , dirt small pebbles etc ... A light weight extruded "billet" type cooler that resembles a "heat sink" with big fins will work pretty good and will be more durable against rocks etc and easier to keep clean ... The very first Lifan "BOAT ANCHOR" ... "fake billet" CAST alloy coolers that weigh a ton and snap frame mounts aren't very efficient ... but they're better than nothing ...
You can buy coolers from ebay of the likes of DHZ or plenty of other vendors around OZ ... just make sure that you don't get sold a cheap chinese cooler at a Kitaco or Takegawa price ...