Nope - it's been done already (compressed air power, that is). A French team tried it a few years ago - on a bike, wasn't it? It didn't work all that well, especially since they seemed to be spending a lot of time at the line "warming" their motor. I thought of that warming as a big waste of their air reserve, especially since they didn't have the vehicle hooked to a compressor in the support truck - or anything.
So if you're going to build for the new roadster sidecar class -- look elsewhere for your power source - unless you're willing to be just another Joe giving old technology a try.