Since I did not have any front fork for the CB125K5 frame I decided to go 'all in' and marry that frame with a CB125TD disk brake front fork.
The obvious was that the distance between yoke and bridge was different. The legs are also longer but that does not matter, I think...
The orther thing was that I did not have any bearings or bearing cups. The solution was to buy tapered roller bearing and lathe a couple of longer bearing cups.
Eventually I will, or not, weld them to the frame for additional strenght.