When I bought my latest cl77 project, I also got in the deal a rusty old P.O.S. of a bike with a few decent parts and a stuck engine. Since the exhaust pipes were full of rust at the bends, I was pretty confident the cylinders would be full of rust as well. I am no stranger to these conditions so I proceeded as usual. So much rust here that saving the pistons and sleeves was hopeless. I am putting this out here just for the info of it all and do not recommend these drastic measures.
First is what I found when I removed the head.
20171104_191956 by
kenneth zukowski, on Flickr
Next I had to relieve the pressure of the rust scale to get these pistons to move.
Note the thin cylinder walls. I/D measures about 65 mm meaning this engine had been bored out to aprox. 350 cc's.
20171111_204929 by
kenneth zukowski, on Flickr
On the good note, this block has no broken fins, can be re sleeved and is very salvageable. Now to dig around the tranny and see what treasures may lie in there. Rusty