base a snpc off a already scripted npc

Is there a way I can base my snpc off of a already made npc like npc_fastzombie? I just want to make a edited fast zombie.

Currently no, because there’s no version of that NPC scripted entirely in lua around (That I know of, and even if there are they aren’t a perfect replacement). Depending on the changes you want to make you could rely on hooks external to the NPC (Which can do things like changing it’s damage, it’s enemies, etc…) or start coding one yourself.

I already have on coded but its navigation and attacking can be buggy and not as accurate as the real one.