If they are the SXS factory forks you are talking about, be aware the 125 2 stroke, 250 2 stroke and 450/525 4 strokes all have different part nos: 3 Different offerings.
I would say they are all sprung and valved for the different weight bikes.
My guess is the 125 fork would require a bit (lot?) of internal tuning to perform adequately in the heavier 4 stroke.
But if the price is right, Go for it !!!