Well, it is an Atari game, so I am going to go out on a limb and say it is an Atari brand trackball. Happ trackballs are basically an atari trackball with a Happ name on them, so if you do a swap it will be a snap. Millipede is a 3" trackball. I don't know what size the bearings are, but Bob Roberts sells them. He also has new rollers if you need those.