I've written all new code for Stern Stars and implemented that code by replacing the M6800 processor on the MPU with a daughter card plugged into J5 of the MPU100. The new code features deeper goals, free play, multiple brightness levels for lamps, scores up to 4 billion, and a wizard mode awarded for completing the three major goals of the game.
The hardware build (less than $20) and game features are documented here:
Here's a video of the Wizard Mode (which demonstrates a lot of the game's features):
I've also overhauled the self test, audits, and parameter control available through the switch in the coin box:
During the process of updating these rules, I created a new Bally/Stern OS that can be used on Bally AS-2518-17, -35, and Stern MPU100 or 200 boards with the addition of a daughter card as described. Calling it a "daughter card" is actually a bit fancy--it's just an Arduino Nano plugged into J5. That whole process is documented here:
The board takes a couple of hours to build and the parts are less than $20 (less than $10 if you buy supplies for more than 4 at a time). I'm hoping some people will give this mod a try on their Stars (it's easy to switch back and forth between the stock game and this mod) in order to get ideas for new features and potentially find bugs.