Yes, nice find. Thanks god, we have the Retro Gamer now also in Germany. So I have learned many about other systems, beside the for me well known videogame consoles like Atari, Intellivision, Vectrex or Coleco. Also the ZX spectrum has wonderful games. It`s always a wonder, what a good programmer could do with so less memory (like with our Videopac).
ZX games like Mazogs, Forty Niner or of course the great 3D Monster Maze (

) are amazing for this little system.
Have fun with it!

You are mentioning some fine ZX81 games here, don't forget to check out my ZX81 work as well
http://www.revival-studios.com/sinclair.phpWhat makes the ZX81 unique is the simplicity of the hardware design (everything on a few chips) which basically lets the progammer do all drawing and handling themselves instead of having a chip (like the VIC/VICII) handle it.
Also, it makes a great doorstop!