It'd be a good idea to look at OSRIC and 13th Age to see how they've done their games. 13th Age has copied several mechanics over from 4th ed, none of which are covered by the OGL, whereas OSRIC outright aimed to replicate 1st edition. OSRIC (along with all the other retro-clones, but I use OSRIC specifically as it's the oldest, most well known and has withstood the test of time in regards to potential lawsuits, which there haven't been) when compared to AD&D 1st edition should be quite telling in what was, and wasn't, copied over and how it was copied over.