On the small CO2 engines the air source has a very small valve with an exposed tip. The pistion hits the valve and opens it. This lets the air in and pushes the piston down. The size and placement is VERY important. You have to time the air you can't just hook up air to the head and make it run. You can use an impact valve or a mechincal one, like a rotary or slide valve. Study some steam engines.
Also lube may be a problem for longer runs