Keeping the small filter / fountain, running will be a big help. I would not worry about feeding the fish, there is plenty of algae and bugs in the pond and you don't have sufficient filtration to handle their waste.
Look to see if there is a koi club in your region and reach out to them for help in rehoming the large koi. The biggest fish will die first from oxygen deprivation and that is a big koi, so please watch it carefully.
You mentioned past predators, can you net the pond so the heron doesn't get them.....with them at the surface, they'll be sitting ducks.
I did mention the heron. It turns up in spring then we don't see it for the rest of the year. But of course I can get netting sorted. Umm the cat's will be cheesed off and thirsty though!