Why would implementing a compatible game engine be unlawful? The code he wrote is copyrighted but the concepts and functional elements embodied by his code shouldn't be protected.
OpenRCT2 and OpenTTD are, at best, derivative works, if not outright infringement (the line between infringement and derivative in computer code is quite muddy, and there is very little in the way of precedence to illuminate the difference). These projects are not like WINE or ReactOS, reimplementing something by carefully observing what it does and attempting to reproduce it, but rather built (originally) by decompiling the original and otherwise heavily reliant on outright reuse of parts of the original.
That isn't derivative. That's a plugin. At that point if OpenRCT2 is calling into the original, intact binary, there can be no infringement. You're just running the executable as provided and your computer just so happens to have another program running in the same memory space.
