Mechanically there the same, as in the con-rods, crank, head, intake are the same. Both engines have oil squirters. The XR-2 has I believe 7.1 comp pistons, whereas the N/A is 9.1. As for bearings and the rest other then the sensors, and turbo and a few other bits it's the same. Theoretically you could buy a engine rebuild kit from Flyin Miata, and all the seals and such would be the same, the only thing you would need to pick up might be new pistons, with rings gapped to whatever size you want, and con-rods depending on how far you want to push the engine.