I can tell you its not worth the hassle to make it work.
Will it work? Of course, but it wont be easy at all. One of the really cool things about the KX250 is how nicely it fits. Its almost like it belongs in there. I remember cutting out the front mounts on my 02 KX250 frame and tossing the engine in there. I was blown away how easily it fit in there and how "right" it looked.
I had a 2004 KX125 I was going to do the swap on, but it just would have been a total PITA. Really the only way to do it would be to do a total custom fabbed cradle. From front to rear, so you're looking at setting up all 3 mounts at that point,
If the 2005 KX125 is a roller, i'd consider buying it off you. I'm looking for a 2005 KX125 roller to swap a KDX200 in.