Not much customization that I'm aware of other than the plastic or wooden grips. Some parts could have come onto the market that I'm not aware of. Mags are relatively inexpensive, stock up as the 5rd rule kind of sucks. Make sure to grease it up thoroughly as I find my Zastava version likes to run wet.
Almost no recoil, so fun to shoot. I shot up to 50yds with mine I had almost no drop and accurate to that distance. Never tried further as my eyes suck any farther than that.
Is the CSA any better or worse than the Zastava? I don't know, haven't heard much about the differences. I'm assuming they're both reliable.
Go have fun!