The banshee ones use megasquirts, then there is one running an old school ecu that uses msdos. There is one being built using a new high tech ecu. There is the technology out there now. You don't need to run an o2 sensor either. The banshee ones dont run them expect to do tune then take it off after initial tuning is done. One guy has many hours on his o2 sensor on his banshee from doing changes on his set up.
The only costly thing for EFI is the ECU. A good ECU is going to cost around a grand. If you know how to use the megasquirt, they are only like $400ish. Injector would be much, just need to know the right size.
For a zilla, you can do a TBI style. Use a throttle body from like a yfzr or ltr or an aftermarket bigger one. Just need some sensor, again not too much.
Main issue is getting some juice out of the stator to run everything, need to make it DC and charge a battery. This is the best way to do it.