Terry Stewart (tezza) talks about a computer in his classic computer collection: The Atari 800
The blog post on the SIO2CD device mentioned is here: https://www.classic-computers.org.nz/blog/2015-06-13-sio2sd-for-atari.htm
The blog post on the The UAV video board is here: https://www.classic-computers.org.nz/blog/2018-03-06-Converting%20-NTSC-Atari-800-for-PAL-using-UAV.htm