There used to be a place in the mid west around Chicago called, I think, Dr. Zipper. they would replace your zipper and/or add a three inch extension via two zippers to your leather jacket.
Don't know if they are still in business but I think I saw an ad for them in Rider magazine last year.

I used them about 10 years ago to replace the zipper on my Heine Gerike Paris to Dakar jacket.