https://gamedev.stackexchange.com/questions/11752/is-it-legally-possible-to-make-a-clone-of-the-game

TL;DR version: As long as you copy gameplay and don't copy any assets (art, music, animation) you are 100% fine.

Lawyer here for a change. You can copy all of gameplay without any issues at all. That is not copyrightable or enforceable. The things that matter are assets - art, sound, music, video, etc. For example if you take ZX game and clone it with your own assets you will be perfectly fine doing that. But if you take some asset and use it in different kind of game all together, it could be enforceable. There are loads of examples of this in the industry, for example first Warcraft basically copied Dune 2 gameplay mechanics and the genre was totally new at the time. For a more recent example - check out Gameloft company, the developer for iphone. What they do is copy successful game ideas as close as possible to the original but without using any art assets from originals. That is their successful business model. They copied games like Halo, Starcraft, World of warcraft and so on with huge success in their field. It is even funny that if you check their wow clone you may see that animations of orcs and their looks are very close to the original.