Easiest way is a freeze plug and a small plate over it. The hole is not threaded, by the way. Pound the freeze plug into the hole (I don't remember the size, NAPA has them for me), then fab a small plate to cover the freeze plug and attach it to the head where the bolt used to go for the air tube. The plate probably is not essential, I always make one just to be real sure the plug stays in. I install the plug with just a bit of hi-temp sealant on it, too.