If you really like the bike and its fork, as is,
consider having a machine shop take the ID of the fork out to 22,2/ie 7/8"

22.2 - 21.15 = 1.05, half of that is the wall thickness reduction of 0.525mm . barely over a half mm..

then the other stems may fit .. the quill of the stem reinforces the steerer as far as its inserted..

the ID gets smaller at the bottom where it joins the fork-Crown.. Aka, Butted..

Another possibility .. there are .833" stem risers *, you could use those, a steel tube
reduced at the bottom, to that diameter. .. to do a conversion to use threadless stem
with shims around the 1" upper portion out to 9/8" to take the common, current, stems..

* that's what is the other equivalency..