When possible I always try to use stuff I have about rather than buy extra things, and it was a spare I got with this digger ages ago:-
The digger as said is a 1972 JCB 3CII, but I'd guess the rad is more likely sourced from some more common tractor or truck from the period. You'll find most if not all modern vehicle rads are Aluminium / plastic so assuming you don't want alu in your system you'd want a rad from a, 1970's era or older car or truck. As you are in the states then I guess muscle car or trucks from the 70's would be a good place to look, you maybe able to pick up a new unit at reasonable cost that is all Brass/ Copper. There are whole industries making exact copy parts for muscle car restoration, or even better someone may still have old stock for a rare or unloved car/truck model. You'll just need to do some detective work to find the right thing I guess. I had to clean the one I used as it was very very mucky, (a brand new one would have been much easier).
I've been getting it ready for a while now as it involved steam cleaning initially then, I ran it for a month in a loop with petrol inside to clean it out, then another month with rad flush & detergents, then a month with water. It's like new now
I then had to solder on the fittings I wanted & make a slight repair which, was surprisingly easy with a small, (blue flame), hand blow lamp. A coat of black hamerite smooth and job done. It seems to be working nice with no fan or forced airflow its keeping the coolant at about 5C to 6C above ambient. I'll try it with a fan when I can find one that is right.