How about something like this? Putting the chain around an idler (which could be as simple as a little bronze bushing on a bolt) avoids the need for elaborate tracks and ensures the chain stays in contact with the sprocket. It should be pretty easy to put a bolt or something through one link and attach it to the end of the piston with a bit of aluminum angle.
Piston and chain mechanism
EDIT: I don't know what's up with that thumbnail, but it doesn't look a whole lot like the actual image...