Could be a sealed unit. You'll know more when you get it out.
Just pull it out, there will be 2 springs that hold it in, they'll become apparent as it comes out. Will be a choc block for the wiring, so just unscrew and put on the bench. Front might screw off to get bulb out, otherwise get a new unit by checking the id marks (serial, part number etc)
You can tidy up the hole by fitting some perspex plates between the light and the ceiling, just measure and search for a size on eBay.
Scrape off the over paint. That would drive me mad. I disconnect all my sockets, switches and fittings if painting and get behind them.