I'm not sure if anyone makes OE replacement pistons for the XR2 - but I know you can run OEM N/A pistons in the XR2 as long as you're running proper octane fuel and aren't boosting it like crazy. The extra compression gives a bump in power and fuel economy. I don't think you'll want to go more than .020 over, the bore spacing won't allow much more, it's pretty tight. Your shop can check that.
That being said, it's probably substantially easier and cheaper to find a junkyard motor with good compression, change the water pump & timing belt, drop it in & be done with it. The NA blocks can be used, you'll need to swap the turbo specific items to the new block but that's not too much work.
FYI if you look for 1.6L Miata stuff regarding the crank & pistons - it's all the same stuff.
