If I remember correctly the 525 is a recent update to the 520 and is extremely rare comparitively as an engine only. Both the 520 and 525 supposedly make more power than the xr650. I'm not sure how common those are anymore either, seem fairly scarce here in the midwest.

 

scarce in Nor Cal too, my neighbor has one... and have ridden with guys that have the bike at the dunes.... the bike is a friggin animal pulls like a 500, revs like a crf, has the torque of the XR.... awesome motor, yea a 520 was a 2003 model and older i believe, which would be fine. cross between a raptor and yfz, and then add 30% more all around when it comes to power.
hahahahahaha