The existing pistons, rods, probably the crank, the headbolts and various other items won't be up to the task.
Not saying the conversion is impossible but it would be exceptionally difficult. For example, where would the pressure pump take it's mechanical drive from? You may also need a supply pump before the pressure pump, maybe not but gravity feed to the pressure pump alone may not cut it.
Zero Pollution Motors have announced they will begin taking reservations for the first U.S. deliveries in the next couple of months, but it will be 2010 before Americans get their first taste of the ingenious compressed-air motor, which runs to 35mph entirely on air, or uses a trickle of petrol to heat and compress more air to reach higher speeds up to 90mph. It'll cost next to nothing to run (how do 30,000km service intervals sound?), have a range of up to 1000 miles, and retail for well under US$20,000.