I use 35 and 50rd Gaili mags in my Saiga, they fit great with no mods. All I had to do was weld a machine screw to the top of the front catch to act has as fed ramp. I had no problems with these mags for about 1500rds now.
I don't remember the screw size. They were in a "junk" bin and they just happened to work. They only drawback is the price of the mags compared to AK or AR mags.