As much as starting over seems like it's going to be a big task (and it is), you can always remember that you have the current version of the project as a very strong base. The writing/dialogues, art styles, templates, maps and plot is all done and it's good, so you can use all that as the base for the new project instead of starting from scratch, which probably will help out a lot.
It's a shame that the coding can't be re-used and it's mostly likely that you will need to start that all over. But at least you won't need to worry about performances issues and bug that happens because of the engine verison difference over the years, so that's that at least. Plus, since you've grow as a developer, this also means you can make the game be closer to your vision.