What size is the freeze plug

Cannot find this anywhere in the owner manual or via a quick search online. It's one of the center ones so expansion plugs won't work (really friggin shallow in there) so we are going with steel, but 1-1/4 is just too big, and 1-1/8 is much too small. I think it may be a 31mm, but don't have an accurate way to measure it. Bonus question, where the heck do you even get metric size freeze plugs?