OP

Quoted from DickHamill:

Meteor2021 - new code for a Stern classic
Over the past year, I've been rewriting Bally/Stern early solid state pinball machines. I plug an Arduino into J5 so the machine can dual-boot between old code and new. I've just wrapped up Meteor2021. It's available from GitHub now.

Why:
The early Bally/Stern games were great shooters that were limited by the software. I wanted to play new rules on my machine without altering the original hardware so I could easily remove my mod when I wanted to move the machine.
How:
The Arduino Nano has just enough I/O pins (18) to interface with the PIA chips on the -17, -35, MPU-100, and MPU-200 boards. The daughter card plugged into J5 (when turned on) Halts the 6800, takes over control signals, a few address lines, and the data lines. Then, the Arduino operates much like 6800 except it's faster and has more code space. The hardware for this project is easily built.
The theory behind the mod is described on my blog: http://ikehamill.com/2020/03/21/replacing-the-m6800-with-an-arduino-nano/
Intructions are on GitHub pages: https://ballysternos.github.io/
Machines:
So far, I've rewritten
* Stars
* Black Jack
* Trident
* Meteor
* (Mata Hari is in progress, but slow because I don't own one of these)
bluebomber has rewritten Flash Gordon
https://pinside.com/pinball/forum/topic/flash-gordon-2021-re-imagined-code-for-bally-flash-gordon-1981
The code is all available on GitHub. It's broken down into a base library and then machine-specific implementations. Rewriting other games requires a moderate knowledge of C/C++.
https://github.com/BallySternOS
Resources:
Here's a suggested parts list. If you bought all these things, you could create 6 of these boards. If you don't need that many boards, you might find cheaper ways to source smaller quantities. I haven't done any work to figure out if this is the cheapest way to source any of this stuff.
Cheap Arduino knockoff x6 ($20.99) - needs CH340 driver for programming / has to be ATmega328P
amazon.com link »
0.1" 40-pin connector (40 pieces for $7.99)
amazon.com link »
32-pin Prototype PCB (2 pack for $9.99) - this won't work for Alltek or MPU-200 because they have a 34-pin connector
amazon.com link »
Wire ($7.99) - tons of wire
amazon.com link »
74125 - ($1.95)
https://www.jameco.com/z/74125-Major-Brands-IC-74125-Quad-Tri-State-Bus-Buffer_49373.html
Boot switch - x2 ($8.99) this switch will work for activating the Arduino board and toggling the speaker (see the writeup here to find out why: https://ballysternos.github.io/install.html)
amazon.com link »
OR
RoyGBev has created a PCB and kit (doesn't include the Arduino) here:
https://pinside.com/pinball/market/shops/1304-roygbev-pinball/04736-arduino-nano-adapter-for-classic-bally-stern