Tempest cabinets were not commonly converted to anything since it required buying a new monitor. They do crop up from time to time though.
Do not buy stuff like this and try to convert it back. If you do then somewhere else someone will part out and torch a perfectly good non-converted color vector game to provide the parts you need. Meaning the world doesn't gain another Tempest, it just looses an original one and gains one that got converted and then converted back again. Except it is a little worse than that, part hoarders and dreamers will suck up a few of those parts and make sure they never go into a machine again. That means for every 8 or so games like this that get "restored", 10 original ones will get trashed to provide the parts.