Shang Starting fresh and porting over the pages one by one would also be my recommendation. There have been so many improvements made in SmartAdmin v4 that it will be worth your effort transitioning to it. Whether this is worth your time we cannot predict nor determine that; it will be your call and judgement to make.
Most of the effort would be to change the HTML markup of what you are outputting as part of your pages (tables, panels, buttons etc.) the rest can probably be re-used (e.g. PHP File structure, code to connect to DB etc).
Could also present you with the ability to do some clean-up and refactoring as part of porting over to SmartAdmin v4.