Here is a list of 99/4a emulators https://www.99er.net/emul.shtml One of these emulators is on github https://github.com/tursilion/classic99 - in active development.

I had the luck of owning a TI99/4A as a kid; it had a very rich Basic, with the 16k extension cartridge you even had sprites and a synthesizer. I have made some 2d game stuff with pygame recently, and for me it felt as if I got back to this old machine... (here is this stuff, for the record: https://github.com/MoserMichael/pygamewrap )