I wonder if one can tune a 2003 ECU to not look for EGR data,
and maybe you can plug the outlet on the 2003's cat converter, ...or just take the 2001's engine headers and its cat converter without egr, and put it in a 2003.
so doing a EGR delete. (plenty of 2003-2008 Ford 6.0 diesels had that done in states where you plug in,...= "bullet proofing" is what they call the EGR delete processes).
if someone were to do it, such would be with free rom-raider, and a tactrix dongle. and if you as a individual did not want to fiddle with it with online help,
maybe a local WRX tuner shop would.
but,...in the end just taking the 2001 engine to a engine repair shop is pretty quick / painless for them to tap out and thread the inlet hole for the EGR. (the space is already there, just not drilled).