There are tons of videos on this subject. One person uses a solenoid air valve that threads in where the spark plug would go. A flywheel magnet triggers the solenoid to open, air goes in for about 1/4 of the stroke length, and then shuts. The inertia drives the crank down and back around. Pretty neat.
Another one is a pop valve. Using a bolt threaded in to the top of the piston, it pokes the valve open when it comes up to tdc. Air goes in, same story.
You could probably make a block off plate for the intake, and fill the crank case with some oil. A slinger on the crank wouldn't hurt.
For all that trouble, I'd stick with gas. It costs way more to refill CO2 cans, and your range is limited by air charging stations. But don't let me stop you if that's what you want to do. It does sound pretty cool, and I'm all about cool factor!