The MX-5's oil pan must be used to clear the subframe. That means ditching the 2.3's balance shafts which are housed there. Clogging the shaft's oil-supply holes will also be necessary.
Since you'll want your 2.3-liter MX-5 to run, you'll need some engine management tweaks. The stock MX-5 ECU should be in the ballpark, and since both engines are mass airflow-based it might even compensate enough to at least run on its own. A little electronic wizardry in the form of a piggyback controller should be able to make up the difference.
So there you have it--a do-it-yourself 2.3-liter MX-5. What? You want a Renesis-powered MX-5? So do we, but as far as we can tell, it's not nearly as easy. Although the MX-5 uses RX-8 underpinnings, the engine mounts are in very different locations and the oil pan will have a territorial dispute with the subframe.
Given these issues, swapping to the Renesis means swapping to the RX-8 subframe, which creates other problems. It's unclear if the frame rails are the same relative height between the two cars, which determines things like ride height and ground clearance. Despite using similar parts, the suspension geometry is very different between the two cars. That means your MX-5 with an RX-8 engine and front suspension could be an evil-handling pig. And then what's the point?
Finally, the RX-8 uses electric power steering while the MX-5's is hydraulic. Everything supporting both systems is different and making one work with the other would suck. Forcing the issue would likely produce questionable results.
Turbos are always good, but in this case present a pile of unknowns. The MX-5's 2.0-liter has 10.8:1 compression, which isn't as appealing as the first-generation car's 8.5:1 squeeze that begged for boost. Plus, it's unlikely the lighter aluminum-block mill will have internals as strong as before.
-- Josh Jacquot