It's not illegal to create a compatible game engine. The functional ideas inside the games are not protected by copyright. So long as games are clean room reverse engineered there should be no problem.
Actually, even if the reverse engineering was not clean room, it might not be a problem.
Sony Computer Entertainment v. Connectix Corp.
> The object code of a program may be copyrighted as expression, 17 U.S.C. § 102(a), but it also contains ideas and performs functions that are not entitled to copyright protection. See 17 U.S.C. § 102(b).
> Object code cannot, however, be read by humans.
> The unprotected ideas and functions of the code therefore are frequently undiscoverable in the absence of investigation and translation that may require copying the copyrighted material.
> We conclude that, under the facts of this case and our precedent, Connectix's intermediate copying and use of Sony's copyrighted BIOS was a fair use for the purpose of gaining access to the unprotected elements of Sony's software.
