Pin hole in cast iron head

That Belzona stuff is probably just as valid as JB Weld (I'd like to know what the differences are). I'm not sure how epoxy would hold up against fuel over time.

Maybe plain old plumbing solder? Water pressure in pipes is typically 55+psi, over a hundred if you don't have an expansion tank. Temp in the head should be less than 300 degrees unless you have other far more serious issues. Plumbing solder melts at 450+