Well, many planes work as AI right out of the box, but some don't. The only way to find out is to try them. Just write the title from one of the User Interface sections of the aircraft.cfg file into a flight plan, go to an airport where the plane should be, and check it out. If it works, then you have a flyable plane that can also be used as AI without any further work. Just write it into flight plans as you wish.
Some don't work because of something about the visual model. You can't do anything about that. If a plane doesn't show up, or isn't visible in all views, you won't be able to use it as an AI plane.
Some don't work because of something about the flight model. You can do something about that. Sometimes all it takes is a simple tweak. For example, AI planes must have a flaps section in the aircraft.cfg file, whether they really have flaps or not. Otherwise the AI engine will try to drop the flaps for takeoff, and when it can't find the flaps it bogs down. The plane will sit motionless until it disappears. You can fix that by simply copying a flaps section from another plane, like the stock Cub.
Sometimes it's not that simple, and the easiest thing then is to just make an AI version where you replace the flight model completely. Find a plane that you know works as AI and has the same number of engines and the same landing gear type (i.e., nose wheel or tail wheel.) Make a copy of the aircraft.cfg and AIR files from that plane, then replace the following sections of the aircraft.cfg file with the corresponding User Interface (i.e. [flightsim.X] sections), Contact Points, and Lights (if present) from the plane's original file. Be sure to make some slight change to the Title and Variation in each UI section so the sim can tell your flyable and AI versions apart. Simply adding AI to the title and variation is adequate. Finally, open the AIR file with AirEd and make sure the aircraft type is 2. That will keep your AI version from showing up on the Select Aircraft menu on the Create A Flight page.
BTW, AI planes all use sounds from the main FS sounds folder, and of course they don't need panels. So you can delete the panel and sounds folders from your AI versions.
Check out your plane in the sim. If it does anything weird, like a tricycle gear plane sitting on its tail, try replacing the Weight & Balance section with the one from the original plane.
If it still doesn't work, maybe the problem is in the model and not the flight files after all...