You can do a Shimano cassette if the derailleur is dialed in at the middle sprocket. That doesn't help with ratio (maybe a goatlink?) but makes sourcing parts easier.
"All our Sliding, Rocker, and PolyDrop dropouts can be converted from a 10 x 135 mm to a 12 x 142 mm hub. It is as easy as switching out the 135 x 10 mm inserts for ones that are compatible with a 142 x 12 mm hub."
May you could drill them and then add brass in the holes with a a MAP burner? You could probably ask in the framebuilding thread where people actually know what they are talking about
My strong conviction after yesterdays commute on a fixed with disk brake fork, is that if it's going to be used on slush-frozen-to bark-like ice and studded tires, you'll be really happy for the rear brake.
If there's no ice in sunny Jylland(?) then ignore above!
He must really like Norway