I haven't seen a working system 1 mpu board in about 8 years. If the machine won't play a game, you can bet the main board is bad. New aftermarket boards are available, but plan on spending $300-400 and a lot of your time preparing the game for the new boards. The connectors will also likely need work, especially if the old battery was leaking. And if he didn't have the keys, I would assume it's leaking.

All that being said, it would be a great game to learn pin repair on as all the parts and info are available. If you posted a few pics, I'm sure someone could help you more with how much to offer.