I don't know Tim. It's kind of annoying to have to get hard to find items like a 21.15mm stem. If that was me i'd pickup a new fork (assuming you don't have a very expensive one already). or ream the steerer.
I'd do 3 if not wanting to spend much money but it sounds like it would take a while. Advantage of ream/new fork is that regular size quill stems are like $10 in a million lengths...
Flip a coin?