These simple systems seem to have some basic map but rely heavily on the O2 signal under load. This means the engine is running at Lambda 1 (stochiometric) most of the time. The downside is loss of peak power and loss of economy at lower loads. The manual is vague on the operating modes.You'd have to learn more about the setup of those OEM EFI systems. My guess is that they are designed to run at/near "best economy" mixture as determined by the readings from the O2 sensor.
here is a link to my B&S EFI manual and the EFI parts listings for the 49 and 61: Manuals