Voice cloning is still kinda early, especially in the open source area. Whatever you may find now or later (it’s a rapidly developing field), the first and most important step you should do is to digitalize the cassettes into actual audio files to prevent further degradation / loss. Make sure to back those files up too, preferably in the cloud I guess.

You should not need anything special to digitalize the tapes either. A simple cassette player with a 3.5mm audio output fed into your mic input on your computer and a recording software such as Tenacity should be enough (start recording on the program, then play the cassette).