This is a tough job to replace or fix an oil cooler in an alum radiator.
Sounds like you have a unit made by a company in Sacramento, CA when Bruce and I were having them develop an alum GN replacement radiator, and the issue was no proper alum coolers were available then, so they had to develop one to fit?
Your options may be to replace the entire unit, or bypass the oil cooler, as it may be too expensive to repair.
Not many cars use a radiator oil cooler, so yours would be worth something in the used market where you could recover some of your investment and put it towards a new radiator.
I do have a hot air car, no oil cooler, I am now working on which I will be replacing the radiator. I do have a 1987 GN radiator with a new high-effiency core, and if this could be of help and interest, let me know.