GW-BASIC is fun. Microsoft released it with a MIT license and then someone forked it to make it compile and actually work:

https://codeberg.org/tkchia/GW-BASIC

For development it is convenient that PC-BASIC exists, that is a pure Python implementation of GW-BASIC that has its own partial 1999s PC emulator built in.

http://robhagemans.github.io/pcbasic/