There was an article in an old trail and track mag on doing this to the mid 80's models. I dont know the issue number though. You just gotta find the right points to tap into the cases for oil lines. Desert racers used to do it on wet sump 250 Motosports, the 350's and even XR75's all the time and fit Lockhart Harley oil coolers.
Pros - cools the oil
Cons - more things to break/come loose/more weight added