Depending on the platform, easy enough to bypass lockout. The traditional way to lock out old consoles is to require you to include some material with a Nintendo copyright or trademark on the cartridge. That way, Nintendo can sue unlicensed game developers for copyright or trademark infringement. Various systems of this kind have been bypassed without the use of infringing material.

The lockout circuit on the NES, for example, can be bypassed by sending it a negative voltage spike. You can put the circutry on your cartridge.