I'm hoping someone out there will know what extra parts I will need to do this conversion,
I can get a hold of the JDM calipers rotors knuckles bearings all made for a 2002-2003 wrx.
Will I need anything else to bolt this on a 2003 wrx?
They should be drop in with the parts you mentioned plus the kartboys rear 2 pot adapter. Are you swapping OEM calipers for Brembos and do you have any aftermarket brake parts already installed?
While your in there you may as well to steel lines and please do a vacuum bleed rather than a traditional bleed. It makes a huge difference especially when your talking about someones old calipers that have been exposed to the humidity in the air.
A couple of things to keep in mind when swapping for larger more powerful calipers, They typically will require more volume of fluid to actuate the caliper and in most cases will increase the pedal travel over stock. I think the 03 WRX comes with a pretty over sized master cylinder already so you will probably be OK. just keep it in mind if the pedal feels like its got too much give. you can always swap for a larger master cylinder but you'll be looking at a 3rd party if you want to go bigger than the Subaru MC.