Until companies like Namco and Atari start selling ROM licenses directly, I don't know that it's possible to be legally clean here, unless you buy one of the packaged cabinets from a manufacturer who is big enough to have negotiated a resale license.
Legal download of Atari ROMs? Download iTunes and Atari's Greatest Hits (both free) Open the Greatest Hits 1.3.ipa file with WinRAR, browse to Payload\Greatest Hits.app\Library\Data and help yourself to a ton of bin files. Don't think any of the Namco iOS titles are emulated - but you can get ROMs from some of the SEGA apps...
It is impossible to enjoy idling thoroughly unless one has plenty of work to do. -- Jerome Klapka Jerome