I think the conversion problem mainly relates to blank firing pistols.
As Sewdhull said, most of the operational components are already present.
They'd have to chop the barrel, drill out the breech (probably thread it) and replace it all with a steel sleeve and barrel part.
It wouldn't last as long as a real firearm since the low quality body metal would probably deform or crack over time.
With a GBBR they'd have to do all that AND make a steel bolt with extractor, firing pin AND alter the trigger assembly to hit the firing pin.
Even with all those changes the poor quality metal body would likely break the first time it fires.
Yes, some AK's supposedly use real steel bodies but the gas tube and block are not. Nor is the gas piston or bolt which would also have to incorporate a firing pin etc...