Reverse CDI (without DET in it unless you want to change the head too)
CDI pigtail harness
Either the reverse handle bar assembly or just use your own momentary switch from Radio Shack
The 03 had the 18degree flywheel and reverse compatible oil pump already.
Oh, and you need a reverse driven clutch/helix too.
I bought a kit for my 03 rmk and it shows other kits available for Xc/prox or rmk's. It shows the Xc/prox is the same kit. All you really need is the Cdi, harness, any button and the proper helix.