You can use a single injector at the intake—Throttle Body Injection (TBI)—to replace the carburation metering with a fuel injector. A lot of domestic manufacturers did this for a time when they were struggling to convert to EFI.

For the MegaSquirt (at least) the default metering is speed-density which sounds like what your Renix is, and this is the minimum needed to run the MS, unless alternatively you go for Alpha-N fuel mapping.

https://www.holley.com/blog/post/fuel_injection_fundamentals...