Unfortunately the video format used, identified by the AMV or by the MTV extension, is out of common video format standards because specifically studied to be suitable to very low-end hardware. In addition conversion software provided with the players is Windows-only (and often a little buggy).
I recently discovered how to use Linux to convert videos to AMV video format. (It's a recent discover only to me since the project is dated 2007)
The conversion program I used is essentially a patched version of the ffmpeg video conversion tool and can be downloaded from here. Once downloaded all I had to do was enable it to execution, rename (just for commodity) and move it in a suitable folder.
mv amv-ffmpeg-linux-i386-20071030 amv-ffmpegchmod +x amv-ffmpegmv amv-ffmpeg /usr/bin/
I also installed a Youtube download tool
sudo apt-get install youtube-dl
So to convert a video all I have to do is to download it
and convert it with these parameters
the video resolution parameter can change with the device display capabilities while the remaining parameters seems to be the only supported by the devices.amv-ffmpeg -i ZOwHiGCzZjo.flv -f amv -s 160x120 -r 16 -ac 1 -ar 22050 -qmin 3 -qmax 3 ZowHiGCzZjo.amv