Stunning game both visually and audibly! Really really impressive for the time constraint :D
Like a couple others mentioned, it would be nice to be able to restart from the keyboard (probably the space bar to match the start)
I would also really like to see some sort of combo, or at least making your punches cancel the enemy attack (could even be a weak and strong attack system where strong attacks cancel enemy ones but weak jabs don't), I think that would add a lot of depth to the game.
Another way to add a bit of depth could be to replace the 3 heart system with a heath bar to match the enemy's, which enables you to make each attack from the enemy have different weights to them, for example a leg sweep wouldn't hurt as much as a ground slam, so it would feel more rewarding to the player if they block a big hit and avoid taking a lot of damage.
There are really endless directions you can take this game! It's a really good base to build a larger project around! I look forward to seeing where you take this :)