So you'd like to make your own cartridge. You'll need a circuit, and a circuit board. (And then stuff to put on it.)
What we've got here is a PDF with a schematic. You can replicate it yourself in KiCad, tweaking it to suit your needs, and then use the use footprints in my KiCad library to lay out the PCB quickly and easily. By going that route, you can use surface mount parts if you want, or remove the I2C EEPROM (or add another one!), or enlarge the board to have even more storage.
Or you can go over to the releases section and grab some Gerber files that you can upload to JLCPCB. You can't change those, though. They're licensed CC BY-NC-ND 4.0, so you can have them made for yourself, and only yourself. No group buys, no taking my name out of the silkscreen and selling them on eBay, etc, etc. If you'd like to use my design for a commercial purpose, I'd be happy to license it to you for a small fee. If you'd like a different PCB designed, I'm available for that work, as well.
Here's the bill of materials for the layout in the Gerber files:
- SST39SF040-70-4C-PHE
- The FLASH memory, same as the CX16 motherboard uses.
- 24LC128-I/P
- I2C EEPROM
- SN74ACT00N
- NAND gates
- CD74ACT138E
- Address decoder
- RCER71H105K2DBH03A
- Decoupling capacitors
All of those are currently available from Digikey, and I'd expect them to be available from most other major suppliers. The 74xx series parts and capacitors are easily swapped out. There are probably plenty of alternative I2C EEPROMs as well. But there isn't a lot of asynchronous FLASH memory made anymore, so plan on sticking with that part.
You could add on some DIP sockets if you don't want to solder your chips straight to the board. But they probably won't fit in a case if you add sockets. Speaking of which they were tested with this case: