corroded battery on old calculator
First, clean it as thoroughly as you can. You don't want corrosion continuing.

Then, a method that I've seen suggested but haven't tried personally is to wrap some aluminum foil around whatever is available from the old terminal. It's said to work well where it works. i there will be places where foil doesn't belong or you cant get mechanical attachment.

Try to avoid high resistance at join. Measuring voltages when operating with new terminal will tell you how much of battery is getting to the equipment.