Here’s a great how to on Instructables on making a pedal powered air compressor, Aaron writes – “replace the electric motor of an air compressor with modified bicycle drivetrain. pedal. keep pedalling…use the existing motor mount to install an axle with one fixed cog and a belt pulley. You’ll need bearings. try a 1/2″ axle with radial ball bearing cartridges. make the bearing housing out of a slotted tube that has a drilled out nut and a regular nut to allow a bolt to tighten the tube around the bearing….” Link.
HOW TO – Make a pedal powered air compressor