I cant imagine doing the whole motor would cost a whole lot. THeres about a billion of these bikes out there and plenty of parts available.
You may also consider trying a industrial supplier for the bearings sometimes you can get a heck of a deal from them.
But in the spirit of the competition.... Break the ring free with a file.... sand down the piston until smooth and throw some new rings on it. It will run. I actually know a guy that did this on the side of the highway during a cross country trip and the bike made it the distance!!!!
The electric ignition sure beats playing around with the points when you don't have to .....