[zare_st]

Romhacking by refactoring disassembly is something that most experienced (decade+) developers do for a grand project lasting years.

You'd rather start here https://www.romhacking.net/games/884/ and read a romhacking guide. The asset editors and utilities might be enough. More advanced stuff by some asm hacking which may come in form of preexisting patches. Usually it requires understanding only a portion of game's architecture and memory layout.

If you're really keen to start such a project, as I would be keen on playing it, you're likely to spend entire first phase development in level editor getting the sprites and maps right.