Crap. This is a problem with no great solutions.
There still are small diameter stems floating around out there. But they are super-hard to find with long reach.
Could you ream out the stem clamp to 25.4? My LBS will do it for peanuts; if there is enough material there to work with.
You can also replace the fork altogether with one with normal dimensions.
"sand a stem down to 21.1"? Good luck. Do a search and see how many people have managed to pull it off successfully. And see what the ordinarily sanguine Sheldon Brown says about it.
jim
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