Nothing to lose here so drain it out , fix the water ingress / radiator etc .
Go to your local gearbox specialist and ask him for around five gallons of atf.. used atf and then filter it well and use it to flush system out four or five times before putting clean new correct spec fluid in.
My father had a large Volvo estate , big petrol v6 and same thing happened to that and we just flushed it twice and replaced with new fluid and it was ok .
We reckon he had been driving and put over 3k on it before it was found out.
He used it for work and drove 40 mile each way every day.