maybe old 250 froks will work, the newer stuff is to big, you can mod it and make it fit but then the rake is wrong if you use newer clamps. I would be suprised if the YZ forks fit the 500 as they are basically the same as the newer 250 KX forks, but hey I've never tried yamaha stuff so it could be.
Motor, what you can do is press out the stem of the 500 and press it into the 250 triples, you'll have to make a spacer for the top clamp