There's a handfull that have done it and a couple of ways to gitrdon.

any ol' cooler will do.
There are 2 ports on the front of the case and the bottom. M14-1.25 if I remember correctly.  
Now the last time I went to the dealer I saw something that just made me drool, and I asked where do they park these at night?    It had I mean has a nice oil cooler on the front, oil lines running down about the right place too.
But what I'm intending to do as a phase 1 project, is to run some copper line up from one port to the other.  And plug in a temperature gage.

In desert heat be sure you're running xW-50 synthetic motorcycle oil.  The cooler is a great idea.  I'd use steel or stainless steel tubing, not copper, except the copper as a development project would be fine.  When the copper has the right bends, any hydraulic shop can bend steel or stainless to match, as well as providing the right fittings.  Call ahead to see if they can source metric fittings, and the metric fittings probably will need metric tubing...all available in any metro industrial center like L.A. and elsewhere.  There's lots of imported industrial machinery using metric fittings.