Marty has a few specific problems.

The floppy drive is an EME-215FS. When you get a Marty from japan you will find the floppy drive not working.

See photos below taken without permission from Caius. This is because the 26 pin drive has a rubber floppy belt that melts overtime. Even with a new belt sometimes the driver IC is burned out due to prolonged effort and use with a bad belt. The belts are usually the same ones used in the MSX floppy drives. You can also use a floppy drive adaptor to use a 3mode floppy drive or even a floppy drive emulator

The capacitors on the power supply are generally running below tolerance since it's 30 years old and it's recommended to replace them.

Tamura Power Supply

Eastern Power Supply

There is also a capacitor near the Yamaha YM3438. It should be rated at 470uf 16v. This specific capacitor is responsible for crackling audio that you may hear with a Marty straight from Japan with no work done on it. It should be labeled C22 on the Marty 1 motherboard but you can be certain by looking near the YM3438.

There is also a battery meant to keep RTC or time. To my knowledge no crazy RNG generators rely on SRAM being OK or rely on RTC working for Marty but I've seen this in other computers. All the RTC does is keep your time for Marty so if you save a game on a save disk the time is correct. Otherwise I believe the time is just off. It just helps keep track of saves.

Finally the last hurdle is that Marty's laser could be dead and there is zero replacement available in the wild. This is a common problem with desktops. Recapping the CD ROM drive never revives a dead Marty drive.

There is also a leafblade switch that tells the computer the lidtray is open or close. These are occasionally to blame for some dead Marty drives since the disc never spins. The leadblade switch is in the photo below with the red arrow.
If nothing else can be done then you will need an ODE from Deunan to revive your FM Towns Marty.