There are no new cylinders, except for a random one showing up on ebay occasionally. I don't think I've ever seen a good usable used one on ebay. They're always junk.
If the rod bearings look OK as far as you can see and feel with the bottom end together, I'd say leave it alone. There can be center seal problems requiring a teardown, but unlikely. However, if there is any sign of crank bearing problems (rust, roughness, etc.), you don't want to risk a bad bearing destroying good cylinders and pistons when it comes apart.