Also curious, what about the legal issues related to porting existing games?
Viewing post in What's considered a port for the compo
The common practice (Devilution, Return To The Roots, Reminiscence) is that the game usually requires the original one for assets to be automatically extracted. I'd tackle the manual retouching of assets in following steps:
- use a tool which extracts original assets
- after extraction, apply diffs with retouching of assets
and preferably automate those steps. That's the safest approach, I think.