The easier solution is to do what some retrogame coders and tabletop gamers have done. Let's say you want to make an improved Alien Breed.
Well, change the title a bit "Alien Spawn", and then make a game that more or less just happens to work the same way.
As far as abandonware, it doesn't exist as a legal concept. However, with companies that don't exist anymore, the effective, actual risk is basically zero.
Heck, music piracy is actively being pursued and the amount of people who get threatened compared to the amount of songs downloaded is so low that even grandmothers get in on the action.
Also, theft isn't copyright infringement. Ironically stealing a bunch of real stuff in a store is generally punished far less than sharing some imaginary computer data.
