I think it should bolt right up. You might have had an issue if you tried to put the 250F forks in the 125 triples (the four strokes are odd sizes compared to the two strokes), but you've already covered that using the 250 triples. I would do some research on what the triple clamp offsets were for each bike, as that will mess with your handling if you change it too much. I think both bikes used 25 MM, but I'm not sure. KXDino knows a lot about offsets on particular bikes, maybe he will be able to confirm. The other thing I would look for is the overall length of the fork, which can also affect handling. If they are the same length, and the offset is the same, the only other issue you might run in to is the steering stem being too long or short. I don't think this will be an issue, as they were very close in age, but it is something to keep an eye out for. You should be able to press the stem out of the 125 and in to the 250F clamps if you do run in to any issues.
Did you get the brake line from the 250F? If not, I would so you don't have issues routing the brake line. Post some pictures if you get a chance.
