I am looking for help with a '72 Motobecane Le Champion stem. I want to put a longer stem on but....

1. I can't find a allen wrench that will fit the stem. It doesn't look that corroded, but possibly the steel has been flattened to block the wrench from going in. What should I do?

2. I spoke with a bike mechanic about the size of the replacement and he measured with digital calipers (note: I suspect the stem is fully extended). At the top it read 22.08, he slid it down and it read 22.2. So he insisted it needed a 22.2mm replacement. He re measued it and showed me the 22.2 reading.

Also, the original owner told me that this is the original stem.
So....How do I get it out? (could it be stuck?) & What replacement size stem should I get (would like 100-110mm).
Thanks