Converting it would not be easy. You can go with an aftermerket ECU and change it to an SEFI, but why bother?
One thing comes to mind, the 1.8l does not have an EGR system. Mazda might have used MPFI to make sure the engine runs on the rich side. Running rich will cause your CO emmissions a little higher. All engines produce NOx. If you put CO and NOx through the catalytic converter, you merge the one oxide to the CO and you create CO2 and N. If the engine will runs too lean, then there is no CO coming out of the engine and the Catalytic Converter can not create the "better" gas CO2. Only NOx will come out the tailpipe.
Many states, including Ohio, will fail a car for too high NOx emmissions.
How do I know about these gasses? Well, my 1996 EGT failed the Ohio ECheck testing. My HC was high, CO was high and NOx was very high. NOx usually means that the engine is running either very hot or lean. High HC means the engine is running rich. So does CO. Causes of a failing NOx is also a cloged EGR system. Since the 1.8l does not have an EGR system, that can't be the problem. I hooked my Laptop to the OBDII Diagnostic port and the engine was running 190 Degrees. The engine isn't too hot. So there was only one thing that could have been wrong. I got a new cat. and the problem was gone. The EGT passed with almost nothing coming out of the tailpipe. It was something like .50 CO to .10, HC from 800 PPM to 100 ppm. and NOx from 1600 PPM to 200 PPM.