Nafun I measured it with callipers. I've googled around a bit and have seen others w/ 21.1 as well and yes this is the diameter. It is indeed a 1 inch threaded fork. The bike is a Cervelo Eyre with an Easton Fork. I have a whole bunch of quill stems lying around and the only one that will fit it is the one that came with it only it does not have the 26mm clamp, it's smaller then that .
Good idea on the threaded conversion.
I've also heard some people will sand down a slightly larger stem to fit.