you can get enough sensors off ebay/junkyard for cheap to do a much better job than a carburetor, especially with an o2 sensor. But it's a notable amount of work to get it to a point where it is doing a better job.
lemme peek at the
obduino code since it has a similiar problem to solve re: consumption from sensor readings..
Basically you want (going with iat and map instead of maf for availability/ simpler driver setup):
rpm signal
intake air temp
manifold pressure
you also need some constants
displacement
volumetric efficiency
and a specific gas constant
from there you can make a pretty good guess as to how much air mass is going into the engine, and how much fuel to add.
And yah, you are gonna need a microcontroller + injector(s) + pumps/relays + other support circuits
the megasquirt might be overkill, there was another atmega based fuel injection system too, lemme see...
http://ecomodder.com/forum/showthrea...html#post61268 but that does timing and has turbo maps and stuff
so, yah it is doable certainly, but not simple by most measures, if you want something that is better (or even equivalent) than a carburetor.