I am currently searching for a radiator core for a Cat 212 Grader. The engine package is basically the same as the D2-5U. This unit has the oil cooler along side so I'm after the narrower version, Pt. No. 5F2141.
If you're out of options you might try Radiator Supply House in Foster, OR. They are set up to sell to dealers but will sell to individuals. The wide range of specialty and out of production products they stock is impressive. They will work with you. I discovered that the oil cooler on my 1948 D4 was the same as a D2 (later D4 coolers and radiators were taller). RMA to get the right part was not an issue. Be prepared for some sticker shock.
www.radiatorsupplyhouse.com
That's a good find BlueOx - I see they have 5U and 7U coolers orderable https://radiatorsupplyhouse.com/products/caterpillar-d2-oil-cooler-oc-3h2169-850584
I have a core that need repair from my 5U - happy to talk with you about it. However, I ordered a new radiator from RSH and frankly I am very pleased with it, well built and pretty fast turn around. I had to do some trimming on the front top and bottom plates, but no big deal. I would recommend that you get new gaskets from Olsen's gaskets, they fit perfectly, the ones that RSH sends are just strips and will not work.