the CETME is the grandfather of the HK91. after the war some german rifle designers made the rifle for the Spanish army. HK later bought the rights to make their own version (HK91/G3). though similar, parts are not interchangeable.
you might be thinking of the G1, that is a FAL rifle
the only thing I don't particular like about the CETME is the windage adjustment. it's a "cammed" front sight. it doesn't give you a lot of wiggle room. the trunion has to be welded in the receiver perfectly straight, if it's off you'll run out of adjustment and still not be any closer to the POA .
I had that happen when I bought a CENTURY CETME back in 1999/2000, sent it back and got a replacement that shot POI to POA without touching the sight.
as for using G3 mags in the CETME, on mine I can use the G3 mags without any issues with jams, misfeeds etc... don't know about the newer ones on the market.
when I bought mine, G3 mags were like 45 or 50 bucks a pop compared to 7 dollars or so for a CETME MAG. much later the market was flooded with cheap G3 mags for a couple of dollars. that is what pissed the HK guys off to no end, they were buying expensive G3 mags. then all of a sudden, overnight, those mags were worthless and they couldn't even sell them for 10 bucks a pop. that is when I stocked up on G3 mags, they were even cheaper then the CETME mags