Regarding offset, no clue myself about that, spacers could be made to suit. The point was if discs were available without drilling the original hub, which I´d believe from looking into Aliexpress.
I am not an extremist on originality of classic bikes - as most will know by now. Just I see the discrepancy in style of assembled components when these come from very different times, like disc brakes on early 50ties classics. I got a genuine 250 mm Fontana on the shelf but don´t use it on a Vincent as it is minimum 20 years too young. It just does not look right on a girder fork classic. Yes, I do lots of mods that don´t spoil the classic looks and spirit - but then, anybody can do as he likes his bike.
Vic