As it has been said many times before. Linda and Ken's customer service is unbelievable and is like no other. They will do anything to help you and that is one of the main reasons I brought a bike off them. Keep up the good work Ken and Linda ;)
Has happened to me before. Try loosening the little nut that's on the back of the brake petal so that the brake has more movement and so the pads aren't rubbing against the disk.
Might not be that but try it ??
When you registered your RX as a 50cc and you had the 140 in it, did you have to grind off where it had 140cc written on the barrel or did you just leave it there ?