anyone know off hand the size and pitch of the plug that does not come with the radiator?

 

i do believe its the same size as the drain plug...

 

drain plug on the stock radiator? its not the same as the plug on the koyo. its larger than an m14.

 

From what I have read on Scionlife that second hole is for a temp sensor for a gauge. Some people have used a flat washer style o-ring, some have used a standard O-ring but larger in size.

If my guess is close, since that is a sensor bung, try a couple of hex plugs with an NPT thread fitting.