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IMHO: Imagine putting a Motolanna type adapter on those caliper lugs. It would like like crap. The adapter and the lugs would hide and distract from the cool XS1 brake plate. You might as well go disc.
What you want to do is shave those caliper mounts off the 35mm forks, and get a really skilled aluminum welder to weld a rectangular stopper onto the left fork. I know this is a controversial idea.
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If you are worried about the feasibility or integrity of welding on a stopper, here's a solution that Jim came up with.
Make a strut that attaches to the axle and fender mounts. Maybe 1/4" steel, nickel plated. Since it's between the forks, it should be unobtrusive.
I have actually seen this solution done once. It was on a top-notch build, was done well, and looked great -- very concealed. Unfortunately I can't find the pics, so you'll have to take my word for it.