Without actually looking a prices, it might be cheaper just to get a new engine that's outfitted from the factory with EFI.
Honestly, I'd probably leave things as-is. I don't know that you'll get any improvement with going EFI other than ease of starting. Downside is there's a lot more to deal with in terms of fuel system maintenance, winterizing, etc.
Won't you need a 02 sensor and a computer controller of some sort?
Not necessarily. A speed density system with canned programming based solely on RPM and displacement can be ball-park close enough to function decently. Maybe not perfect power or economy, but it will work. You will have to add something that provides an RPM signal. A self-learning system with an O2 sensors like you're talking about is preferred, though.
Brad