I don't think there is custom or modded firmware for avalon units and I never heard of any release a firmware that could run only 2 hashboards the only available firmware you can download is from Canaan.
You don't have any other choice but to buy a new hashboard if one of the hashboard is dead or look for a repair center that knows how to repair ASIC units.
Check the site below for a list of repair centers and maybe one of the centers listed there is near your location.
-
https://www.zeusbtc.com/ASIC-Miner-Repair/Scroll down to the middle to find the list of repair center.