The one with the L0 ROM issue could be simpler to repair given a lack of tools.
Go to
https://m.coolrom.com.au/roms/mame/8917/Neo-Geo.php and download that neogeo.zip file, take out the 000-lo.lo file
Take that file and go to buyicnow, buy a 27C1000 chip with 1x EPROM burning service, email them the 000-lo.lo file from earlier.
While you're at it, may as well burn yourself a diagnostic BIOS too. Buy a 27C1024 (sp1.ROM) and a 27C010 (m1.ROM) from buyicnow and get the diag ROMs from here:
In a dire pinch, you can take any MVS game from about 1992 or later, desolder the M1 ROM and temporarily solder in the diagnostic M1 ROM from the above link and use that to diagnose a Z80 error.
For actually repairing the L0 ROM issue, you use the burned replacement to see if that fixes the problem. If the problem remains, then you look for board damage and corrosion.