No modern dynamo hub will come spaced at 90mm, and I'd be doubtful that even older Sturmey units came that narrow. Shutter precision/Kasai and Shmidt/SON both make a very narrow unit intended for Bromptons and Dahons and other kooky folding bikes, but I want to say they are spaced at 70mm or 76mm, and would build a terrible wheel if laced into a rim bigger than a 16" because you'd get almost no spoke triangulation due to the hub flanges being incredibly close to one another.
There may be a few out there (the cheaper units from Sanyo, Shimano and Panasonic come to mind) that would allow you to get crafty with switching out locknuts and maybe even cones to narrow them up some, but I doubt that would buy you much more than 5-6mm. Any time you do hub work with a Dynamo you must be extremely careful, as most of them have very fine wires that come out of the coil and run in a small keyway cut into the axle to the plug on the drive side, and these wires are incredibly easy to destroy and virtually impossible to fix without a hub factory at your disposal.
Honestly though, if it was me, and your fork is steel, I'd just cold set it to 100mm. If you have a good LBS nearby that is known for quality repair work, ask if they have a fork alignment gauge, and inquire what they charge to do it right, where I work we charge $45 for a bare fork and $60 if we're pulling it out of a bike because then we're overhauling a headset too.
If you can get your hands on an old Sturmey Archer Dynohub, they are actually pretty cool. You can service them (it's a real pain!), they look classic, and to me there is something kind of awesome about 2 different sized flanges on a hub that is just sweet. The biggest downside to those is that you have to lace them tangentially (no radial), and the small flange is difficult to lace because the cutout makes it tricky to get the spoke heads to stay in until there is a bit of tension on them, best to lace all at once with the wheel laying flat, that and the thin steel flanges contribute to spoke breakage at the elbow, so 1-3 brass spoke head washers can be your friend.
Good luck with your project, and LMK if you have other questions, hubs and wheelbuilding is one of my favorite bicycle past-times, and I've tried and failed and succeeded at most bad and good ideas related to it!
