IMO - here is the best way to convert any ICE to compressed air (theoretical - I haven't done it, but i'm sharing ideas for the benefit of anyone wanting to experiment).

Take the head off your existing ICE and throw it, and the fuel system, and the ignition system, away.
Replace the head with a piece of sheet steel - i'm thinking 20mm thick.  Drill holes for the head studs and a hole over the centre of each cylinder bore. 

Screw an electric solenoid operated pneumatic valve into the holes over each cylinder.  Using pneumatic fittings, create a manifold and connect these up to your compressed air cyclinder. 

All you need to do now is wire up an electric control system that will open the solenoids for a specific amount of time at the top of each power stroke.  Speed will be controlled by how long each solenoid stays open.  No air is wasted - and since we have thrown away the valve train - we can operate on all downstrokes, turning 4 stroke ICE into 2 stroke.  OR maybe have the electrical option of only using every second downstroke as a means of lowering power and saving air.

Of course - this is very much under-unity.  But the benefit is we could retrofit this to ANY ICE - petrol or diesel or gas - it doesn't matter. 

We can develope alternative energy solutions for compressing the air at home.  I'm favoring water arc explosions as my prefered over-unity solution to charging compressed air cylinders ...