So you can get the ring on the piston but can't get it back in the cylinder? Try putting it in as far is it can go, and slip on a large hose clamp that is big enough to go over the piston ring. Crank down on the clamp, compressing the ring slightly. (NOT TOO HARD!) Then push the piston into the cylinder (the hose clamp will push off as you do this)
I have used this techinque on small engines, but never an oil free compressor, so I have no idea if this will work or not. It's worth a shot.
Keep me posted...
