I also put the whole Cooper S body kit onto my One. I loved the look of the S over the One but didn't want to be an S. See here for my Old One:
http://www.mini2.com/galleries/showp...6/ppuser/15024
as you can see I left my exhaust alone and just swapped the plactic trim from my original bumper and put it on the bottom of the S bumper. Is your One a facelift MINI or a pre 54 reg? The plastic trim along the bottom of the rear bumper changed on the facelift bumpers and can't really be swapped. Research a bit more if this is so! Hope this helps!