Some details copied from the Discord server which might help. So yeah you can add your own touches and mechanics. You're not meant to copy the game 100%. Add and remove bits as you see fit.
What is the jam about and what do I make?
Welcome to the retro jam 2021! This is a jam where you take a pre-existing game, and fit it in the style of a retro console! It's all about getting creative, or being more unique than just changing the colour palette. You could remake a pre-existing game, such as Portal, and place it in a SNES console's style, making it 2d, or redesigning some levels. You should not only convert the game into a different style, but also add some of your own stuff! You don't have to strictly stick to the limitations of the console, or the limitations of your game, but if you have the main idea there, and it is recognizable enough, then you're set!
What does redesigning the game mean?
So you have to create a game, based around the features of an already existing game. Say, if you did portal, it would be like a new character, with an obvious portal-esque style, and with a portal gun, but you should add new levels, some new mechanics etc.
This is tricky, because its very subjective, but it should not be a 1:1 copy of the game. Change up some stuff, add or remove some stuff and just be creative with it