I do not beleive that the oil cooler was stock on Cali bikes. I just bought two Cali parts bikes and neither of them had an oil cooler and both had oil lines that were not tampered with. I think they would be more readily available if they stock on some model. I think they were a factory option whihc almost NO ONE bought.
Several years ago I bought a used OEM oil cooler for $150 bucks. Let my friend run it on his DR (he did pay me for it). I thought it was pricey at the time, but dang, $275. I still think that i can plumb up a cooler from another bike fairly easily.
That one ol boys oil cooler kit looks good. But we don't have a XR. I think one would pay the top dollar because of the fit and finish, of the mounting brackets, not for the cooler parts itself.
I have not ridden my DR in months.