The update, called Jarvas Redux, aims to stop all known crashes, improve the frame rate, fix background collisions, and balance the difficulty. It also changes a disliked feature where players had to use experience points for magic. Now, there's a new "MP" stat that refills with items.
Here's a short description from the project page:
"You're an astronaut who lands on a future Earth after an apocalypse. Your job is to find allies and attack castles to defeat the Evil Monarchs ruling the seven continents. But the game's biggest challenge was its constant glitches, many crashes, trial-and-error gameplay that often sent you back to the start, and sudden difficulty spikes. Since the game is known as one of the worst on the Famicom, hack creator Mentil tried to make it playable and even fun."
The description also says Mentil hopes this project shows that "no game with good basics is beyond fixing". They want to inspire other hackers to improve similar games that just need some extra work.
You can download the patch here, if you want to give it a try. It is worth noting that it is compatible with both the original Japanese ROM and the Stardust Crusaders 1.00 English patch.
Jarvas Redux (NES) Romhack - RomHack Plaza
As an astronaut arriving on a future post-apocalyptic Earth, it's up to you to recruit allies in order to storm the castles and overthrow the Evil Monarchs
romhackplaza.org