I wouldn't recommend installing a '75 CR125 front brake on anything.
I have the same hub/brake on my MR and it works sort-of OK with high-friction linings, but I replaced my CR's brake with a double-leading shoe setup and now I can actually stop quickly.What I would try to find is a '76 MR250 front wheel and brake plate. This 160mm drum works really well - I was able to lock the wheel at 40mph on pavement without much effort - and should be easy to fit to the CR250 forks, provided the backing plate slot lines up with the fork stub. The total wheel/hub width (with brake) is 107mm/4.2" without the speedo drive, so it should fit between the fork legs with room to spare. The MT250 wheel/brake is pretty much identical to the MR250 and should be a lot easier to find, but it has a steel rim, if that's a problem. If you have the old CR250 wheel, you could always lace up that rim - I did this recently, it really wasn't that much work at all (the spoke nipples weren't all frozen up, though).
Ray