Quoted from pinballwil:Afm and MM are non licenced game's.
So he can make a Martian Attack or a Mediëval castle or something whitout having licence problems.
The original did not need a license, but I would think any “sequel” where you want to re-use some artwork or call outs would probably need a Williams or Midway license.
Sure a “new” original medieval or space theme could be done without license...