I just received a care package from my father with a pile of old "junk". In there was a Speak and Spell from around 1978, I don't know when mine was bought.
I quickly checked the battery compartment (4 C-cells) fearing to find a pile of corrosion. I did, but it was the rusty sort. The battery contacts were rusted.
I opened the unit, removed the contacts and dropped them in vinegar to dissolve the rust. The contacts completely fell apart.
A quick Amazon order for Keystone 209's and I was back in business. The new clips seem to be slightly thicker or perhaps stiffer, as the batteries are more difficult to insert. BUT it works!
While I was waiting for the 209's to arrive, I considered adding an 18650 battery and a USB battery charger. The old TMS5100 series electronics runs from -15v so there is an inverting boost regulator to drop the 6v down to the -15. I tested and this works OK from 5v which is the standard output from the variety of battery manager/chargers you can find. But in the end I did not want a second button to activate the USB battery and then the normal ON button to activate the device.
In any case, my 5 year old son loves it, though he did ask "can we install more games".