"...I've thought quite a bit about doing more work on it..."
"...whether to keep hacking away at the original or just start from scratch..."
Knowing next to nothing about programming, I can't begin to give any advice. But based on other games that have been re-written from scratch (i.e. Donkey Kong, Pacman(s), Midevil Mayhem, etc), maybe the chances for the best results is for you to start with a clean slate. Bottom-line, either way, I'm glad you're giving it a try.