Fighter planes are high performance machines. Things tend to break. Lots of spares are required to keep them operational in peace time, let alone when they go into combat for real. Naturally, these parts are not something you would go pick up at the local hardware store or order from Amazon.
I'm not sure McDonnell Douglas (now Boeing) still supports these aircraft, nor if they do, would they be inclined to for a country like Iran. More and more of Iran's F-4s are no doubt being cannibalized to keep a shrinking number airworthy.
No doubt the Russians or perhaps the Chinese could modify current technology to function in these elderly machines, but that would require a team of engineers to get the hardware and software created, installed and extensively tested. Very expensive to do something like that.
There's a big difference between getting something just barely flying and having it perform in aerial combat. Israel would sweep them from the sky or blow them apart on the ground. They would have to run away to Afghanistan and hope they could land somewhere more or less friendly, much like Iraqi jets did in Desert Storm.
Personally, I'm surprized the Russians/Chinese haven't supplied them with MiGs.