Very large 30 yo houseplant is root rotting. What's the best thing to do?

This schefflera is probably 30 years old and weighs 100+ pounds in the pot. It was the healthiest ever this summer as we bring it outside for the season however the rain has flooded it. We poured off as much water as possible (The pot has no holes). Now it’s back inside, November, everything is wilting. Should I attempt a re-pot? Not an easy task as big as this plant is. Should I trim off this year’s growth? Everything looks deathly on it and I wonder if it can even be revived. (I really didn’t want to drill holes in the pot now after decades.)