Due to the scarcity of 90mm front hubs how much of a issue would it be to "spread the forks 10mm to accomodate much more readily available 100mm hubs???

Do it really slow and put something in the middle of the fork blades to it spreads from the middle down (a belt or something) and not from the crown, you really dont want the fork blades to crack up in the sockets area.

Depends on the fork. Prolly not an issue.

Although I'd be inclined to just jam the wheel in every time rather than cold-set the fork, because it's hard to check it for alignment.

Moving fork blades is much more difficult than adjusting rear stays. The blades are very stiff, and you need to remove the fork from the fram and secure the fork column without damaging it in order to move them evenly. Also, in my opinion it's not advisable to spread fork blades without dropout alignment afterward due to the greater stress from the blades and the smaller diameter of the front axle. Others may think (or know) that it's not that big a deal, but I believe it's best to remove the fork and take it to a shop for proper resetting.

I would absolutely NOT attempt to bend the blades "from the middle down" - that sounds like a recipe for disaster to me. Again, anyone who is experienced with frame alignment is welcome to chime in.

What type of fork is this ? Could you replace it with a 100mm fork, or is it something special and unique ?

I don't have any experience, only the thought that a front fork failure can be catastrophic. I'd be surprised if any shop would do it.

Ditto +1. I've done it (grab it like superman unbending a steel bar and pull) for slight spreading (4 or 5mm) just to allow an easier squeeze, but forks are dangerous things to compromise. Then next time you're in tuck going down a steep hill at 40mph+, you'll be very worried about front end integrity.

how about modifying a 100mm hub axle? you could cut the axle and change the spacers to make the OLD 90mm. that would be assuming that the hub/axle you have has the appropriate amount of spacers to allow it. some hubs are wider from flange to flange and use no spacers, those type would probably not work.

for all of the hassle involved it might just be easier to get a new fork. based on the 90mm spacing i'm going to assume it's for a 27" wheel and it's nothing special. this may fit the bill, it's spaced for a 100mm hub, and it's cheap: https://www.amazon.com/Sunlite-Thread.../dp/B0014C639K.
amazon doesn't have it, but your LBS can probably get on through QBP or something.

An inexpensive chrome replacement fork may be a good idea - realignment of your current one may cost as much. I would have slight concern about moving each blade 5 mm out but I think it's doable. The Amazon fork seems to be unavailable and we don't know if the OP needs a 27" or 700C fork, and headset seat compatibility would be another concern.

Or modifying a 74mm wide folding bike hub.

thing abour the job you describe is the fork can be bent in several planes and you can mess up steering,
without the accurate measurement referance to keep it alligned..

No doubt that a compromised fork would be catastrophic. I have first hand experience,. Many, many years ago I road a bike down concrete steps at Pensacola Naval Air Station, not once but three times. It failed on downstep run #3 and I did a face plant on the sidewalk. I will be replacing the fork with this: