Jailbreak, install Cydia, find emulators. You can buy a cable to hook your iPad up to a TV, and there's yet another Cydia app that will enable it for everything (IIRC, Apple restricts TV mirroring to a few apps). Use bluetooth controllers. At one point, someone sneaked MAME into the App store and it wouldn't surprise me if you could find it digging around.
I messed around with it but whole endeavor seemed pointless.
All of this is very easy to do on Android devices, of course.