Back when these radios were being built, having a tone function was an option, not a standard feature. Kenwood offered a TU7 tone module. It was installed in the bottom of the radio, an easy process. all you had to do was buy the module, remopve the four screws holding on the bottom panel, plug the module in, and secure it with two provided screws. Replace the bottom cover, and you're done.
This ideo shows making contact oon a notone repeater, as well as a tonerepeater.
This is available for sale on FatCat.Parts. Well, to be exact, it was made available the day this was published, so you'll have to check out the website to see if it is still available.