I just replied to a PM with my yz125 caliper info figure Ill post it here to since I dug it up from a post I made on 3 wheeler world in 2012 (different username)
Cost was 29$ off ebay for a 1999 yz125 front brake caliper with good pads used--caliper did not need any shims at all, just had to slot the bracket holes and remove some meat from the bracket under the caliper to give the rubber boot on the caliper a bit more clearance--the caliper going from the front of a yz to the rear of the atc is still stopping in the same direction-(yz front left flip the caliper and it goes on the right rear same direction stopping so your golden) there is no parking brake on the yz so no need for a block-off(at least I always remove the parking brakes from my quads) they mount up clean and there is a very little amount of dremal work needed on the 250r brake stay if I remember right I split the difference in the front and rear holes on the brake stay maybe 1/8" per hole of meat removed so nothing major at all and yz was a duel piston caliper
here is a picture of the 125 caliper on the atv---the ATC was a 1986 250r