You can charge for an emulator as a service for building and packaging it on the App Store (assuming you have permission somehow) or as long as you can definitively prove that you built it via some "clean room design" - that will protect you from legal issues should anyone decide to litigate you for stealing copyrighted code. Backward engineering something (an emulator) to run software (a game) is not illegal, as long as said software doesn't run copyrighted code. This is how WINE exists, after all. Yes, I know WINE is not an emulator. It's a compatibility layer.
Will anyone litigate you for a DOS emulator at this point? Likely not.
Will anyone litigate you for a DOS emulator at this point? Likely not.