It doesn’t have to execute itself on the onboard system of the plane, it simply has to have control of the remote control system.
Planes that are primarily designed to be human-piloted tend to have to be wildly modified to become a drone, or a remote-pilot situation. The F-35, for example, can be heavily modified for this, but is not built for it to begin with. This argument would hold more weight if you were referring to the entire drone fleet.
(Assuming drones) Generally, the military is pretty secure with these kind of things, and they won’t allow in external internet connections but will instead have their own internal communications network. For this to be successful, the AGI would essentially have to somehow get by air-gapped defenses and get close enough with a physical body to get a signal. How could they do this with drone pilots in a remote area in an non-internet connected building? The only way would be through the wireless signal. At that point, yes, it would be feasible to take over the drone. I find it very hard to believe that an AGI could do that, magically make connection to remote, air-gapped systems.
An AGI doing what you’re talking about doing would mean all secure facilities in the world would have just tossed their security practices out the window to begin with and having internet connections inside secure facilities. That’s just not how its done. Sure the psychotic wing of the Republican party doesn’t give a shit and Donald Trump doesn’t… but like, reasonable people do, and so security still exists.