@Morten
I’ve got Yz125 UD forks on a DTR125 frame. The previous owner fitted a Dtr headstock tube into the lower yoke (Yz ones are an inch or so longer) It’s not a perfect job though cause the yokes don’t line up with the ‘hourglass’ shape of the fork legs...
Best machine lathe a longer cup bearing adapter out of stainless and press it into the headstock IMO 😉