Kung Fu Flash can emulate different cartridge types, load PRG files and use D64 disk images. Just place your desired CRT, PRG, T64 or D64 files on a FAT formatted microSD card and use the built-in launcher to execute them. More info in github link.

Assembled and tested, micro SD card not included, USB connector on PCB so you can easily update firmware in the future.

You can print your own 3D enclosure (use the files for rev.2): https://www.thingiverse.com/thing:4153414