Confirm first if the zipper is not the original itself...usually you can spot some evidence if you study the stitching, you might notice the thread color and lines are not a match to the original thread holes if the zipper has been replaced. if It's the original zipper, it's hard to say whether to leave it or install a new one...depends if you really plan to wear the jacket or not I guess.
It's a bit of a collectors item at this point I would say.
And if you don't know a part, or name of a part, or part of a part, just look it up: