Pin hole in cast iron head

16psi times a hole .020" in diameter is like .005 pounds of force, if my math is right. There's not much force on that, at least from the coolant pressure with a 16psi cap. I would think most epoxies/bottle welds/brazes would seal that up.

Someone should check my math. I don't feel well today. Disclaimer, I'm just spitballing the hole diameter because OP doesn't say it.

Pour coolant on it and see if it drains out.