IF you know what the particular model looks like unmodified, then it would be a good idea to restore it as much as possible. That being said, you need to be sure you can do a good job or find a gunsmith who can do it right. While this looks like a New Model Evans, it might be a Transition Model (the Old Model has only wood above the magazine tube). In restoring, DO NOT reblue the metal, or re-finish the stock! That kills value as a collector's piece. If you can accurately determine the barrel band size and shape, and find something close, that will help. I doubt you'll find an original.
As far as "how much" the Evans is worth as-is or restored, you first have to find someone who is interested in the gun. If you sell it to a dealer, remember, he/she will offer "wholesale" so the gun can be sold "retail". If you find a private individual who is interested, it will be whatever you can negotiate. I would say the gun would probably be worth at least $800-$1,000, but it might be worth quite a bit more. That is NOT an offer, BTW. While I find the Evans interesting, my "exchequer" is way down right now.
Best of luck!