As red58 said, you can back flush it. Just be ready. Sometimes they will leak. But certainly worth trying. If it does leak, take it out and have a radiator shop fix it.

If you get lucky and it doesn't leak, you'll have heat.

I wouldn't turn full hydrant pressure on it though, a little at a time to get things flowing. Then you can allow more to go through.

But if you hit it with full hydrant pressure, you may be assured of a leak. Those old cores only had around 7lbs of pressure on them.