This project exposes a 16 KiB window of a Raspberry Pi Pico's RAM to a Commodore 64 via the expansion port. It includes a hardware design and software for the Pico C++ SDK.
 
5V power from the expansion port is used to power the Pico. As suggested in the Pico datasheet, a Schottky diode allows the Pico to be connected to powered USB at the same time.
8-bit buffers bring the 5V logic levels on the address and data bus down to 3.3V to protect the Pico's GPIO pins. This means that when the C64 reads from the Pico, it's putting a 3.3V signal on the data bus. At TTL logic levels, this should be fine.
A switch allows you to toggle between an 8K ROM and a 16K ROM by pulling the /GAME line low. This loses half the address space, but lets you boot into BASIC.