Put the mainboard into 2-propanol alcohol for at least 24 hours.

Then put it into fresh 2-propanol alcohol for another 5 hours.

Put it near (not on) a heater/radiator and wait another 24 hours.

(While waiting clean corroded contacts from the components in the housing/case with a toothbrush and 2-propanol - very gently, not like scrubbing your teeth!) :)

reassemble and pray. Good luck!