I had a link, but it's dead now. I printed out the instructions a few years back but I can't find them at the moment.
As I recall, it was just a matter of some minor dremel work on the MG-13 mag, and I think the Hakim mag catch has to be modified a tiny bit. I remember thinking that I could have probably done the mod just by looking at the original mag and grinding the MG-13 to match. It looked very simple.
A big mag is nice, but if you want a Hakim with slightly LESS firepower, look into the single-shot 22 cal airgun Hakim trainer made for Egypt by Anschutz. I have five of them...
They even come with a cool flaming skull...