I agree with Ross. I can clearly see the cast boss that the generator stud would thread into, but there is no stud showing in the gap between the manifold and generator bracket. Either your stud came loose and backed completely out or it tore the threads out of the aluminum manifold. Looks like you’ll need to repair (heli-coil) the threads in the manifold and replace the stud and radiator. Rough day, brother!

On some aftermarket aluminum castings, the manufacturer will install a stainless steel threaded insert in all the threaded holes as the part is manufactured to increase the strength and longevity of the threads. I would suspect that they did not in this case because of the size of the threaded hole. That doesn't mean that you can’t do it yourself, though. And given the amount of wreckage you're looking to have to replace, a Helicoil kit to help prevent a recurrence would be a wise investment.

I now see that I misread your question - the stud itself should be steel and you did in fact find the remains of it. The nut and square washer are distinctive. I was referring to the threads themselves in the intake manifold being a stainless steel insert. I don’t believe that I would use a stainless stud in that application. Stainless bolts and studs can tend to gall and smear their threads. I’d replace it with a new high quality grade 5 or grade 8 plain steel stud. Unfortunately, the condition of that hole and it’s location may force you to take the intake manifold off of the engine to get the rest of the broken stud out. I have a bad feeling that the first couple threads are deformed enough that it will put up quite a fight. It may need to be drilled out.

That poor radiator is going to have to be recored. I’m not sure what you have available as far as radiator shops go over there, but in the US, a recore job costs about as much as a brand new radiator. The all aluminum units are quite a bit less expensive, but they kinda look funny attached to a traditional flatty in my opinion.