Actually, this is probably really hard to do. Last time I saw inside an ordinary, non-scientific calculator, it was just a board with a little resin blob on it, everything was in there.
You may find it works much slower is you lower the supply voltage. Some seem to, some dont. So, it may be as simple as placing a resistor after the battery.
It certainly would be funny, if it worked :lol: , give it to someone to borrow, then see how long it is before they give it back; and try and find another one.