If the board is the same, I will let you know what I find out when it gets here. Your board might have a clock doubling circuit to simulate fast. A scope on the clock input of the CPU might give you a clue or maybe a clock divider chip might be it. Hard to tell.
A very long time ago, I use to run a surplus eletronic shop and was offerd a bunch of clone ms pacman games board. They were all were slated to be crushed but I think a few got back into the market so I am a little curious about the board I just bought.