Marty is known to have a terrible FDD. Even with belt replacement it's very common for the drive will be able to read but can't write data which impacts things such as saving games.
There are a few methods to use a floppy emulator with Marty.
HXC method via Bit-District
Bit-district makes a nice tray that is meant to be used with DocBrown ODE.
Note you must use the HXC Rev C model only. It's recommended to buy the floppy emulator first, measure the buttons and LCD and then buy the appropriate tray configuration.
You will also require a 26 pin FFC to 34 pin IDC converter to use the HXC properly inside Marty.
Caius has made a totally custom PCB variant of a gotek floppy emulator with flashfloppy firmware. It is plug and play and variants work for Marty with the original CD drive or with ODE.