Fortunately, if the front bearing housing is detachable, you can remove it along with the crankshaft and then slide out the sleeve - with the piston inside it. Then, you can access the port where the ring is hanging and hopefully press it back in to remove the piston.

If the crankcase is one piece, and the ring is hooked, there's not much you can do, unless it's hooked in the exhaust port.

David