There are radiator repair shops that still do these types of repairs even on today's newer aluminum and plastic radiators. Locating them and getting slotted in their schedule is generally the most difficult part. They're not as widely available as the old radiator repair shops that did copper and brass repairs, so depending on where you are it might even require shipping your unit to them.
I personally don't like adding things to stop a leak, but if I did, I'd not use any form of "stop leak" additives to attempt the fix on my tractor. I would likely connect the radiator to some other pump/reservoir setup to run the stop leak through it just to keep that stuff out of my pump, thermostat, and the passages within my motor. I'd also recommend heating the fluid in your make-shift reservoir to simulate the heat of a running engine to ensure the product will work as described.
I've heard of several things you could add to a radiator to stop a leak ranging from eggs to black pepper, but if I was doing it myself I'd use a product made for such a thing, I just don't think I'd want it all throughout the system.
