There's arcade stuff about for sure (dirty cigarette burnt cabs and cockroach infested PCBs), but it's not readibly accessible to the public. I'd hit up LegendHK and maybe even some English friendly vendors such as Tototek before leaving and ask if they can help you out with some connections.
Here's a list of things worth checking out:
Retro Game Stores: Ho King & The Sino Centre (Basement Store) in Mong Kok
Oriental 188 Shopping Centre in Wan Chai
Golden Computer Centre in Sham Shui Po
Video Game Arcades: Smart Game & Game Zone in Mong Kok
Plaza Hollywood Game Zone in Wong Tai Sin
Game Centre in Central
X-Land in Tsuen Wan
American Pool in Whampoa