Being unfamiliar with MTB and BMX stems and forks from that era, what's the
outside diameter of the steerers that take a 21.1mm / .830" stem?
Easiest way to tell is what's the thread size of the headsets?
I don't know the OD, yet. i'll have take things apart and apply the ol' calipers. hopefully later today (i'm up way too late...lol)
If it's 1", the steerer can be bored out to 22.2mm on a mill or a lathe. Trying to ream out 1.1mm is a LOT of work. Reamers are intended to remove a much smaller amount of material to accurately resize a bore.
The fork would need to be rigidly fixtured on either a mill or lathe while boring it out, otherwise the cutting tool can easily grab inside the bore and you have a mess! (Been there - done that).
Several other things: the bore should have a taper at the bottom of the resized area rather than a sharp shoulder to reduce the possibility of a stress riser and... make sure that the expander wedge is below the threaded area of the steerer. 75mm to 80mm is the recommended minimum amount of insertion for the stem into the steerer.
hmmm...i'm thinking blow off mod'ing the fork, then. that sounds....tricky! i'd literally jump off a bridge if i messed up this fork. it's one of a kind. and, i suppose i don't need to go messin' up my pretty face on the pavement
There are a number of European websites that list the Humbert Dino-X 21.1mm / .830" Stems pictured above. They all appear to be out of stock except for maybe xxcycle and the info goes back to 2019.
I wonder
how strong the cast aluminum extensions attached to the steel quills are on those Humbert Dino-X stems. Probably OK for around town use but I wouldn't feel comfortable with an average sized or larger adult using one of those off road.
it's a good question. it's how my ross stem is constructed, and humpert is a brand i'm familiar with. over the years i've seen the name thrown around particularly in the euro touring groups. so, perhaps it's quality despite unfamiliarity.
Also, I wouldn't recommend removing 1.1mm+ off of the diameter of a 22.2mm aluminum stem like those tall boys from Nitto. The wall thickness of the quill would probably end up being too thin.
yeah, i know...that's a crazy idea. but, i've seen some pretty thick alloy quill inserts. they're heavy! i've a promax like that, but it's also 1 1/8. probably best to blow off this mod'ing idea, too
Here's a suggestion: get an old steel 21.1mm MTB stem with a brake cable roller that you like and have an extension TIG welded to the bottom of the quill???
now, that gives me an idea!!! but, before i expound upon it here, i'll have to take some measurements of potential frankenparts beforehand. i'll report back on this 
Back in 1991 I built this lugged Reynolds 531, 700c off road trekking frame. I had some neck problems and Salsa made me this short reach, high rise stem. Solved a bunch of problems!
i've seen that bike before!!!! i ran a google search on "short quill stems" and a BF thread popped up with that and several other bikes of yours shown.
One last thing, I just saw these Cro Mo steel stem extensions. Dirt cheap!
Raised mtb stem on an old '71 Super Sport ?
i didn't see much helpful info in that thread, but i know of the cromo extensions you speak of. my above reply of "frankenparts" will include one IF measurements pan out fortuitously
verktyg
