Anything can be rebuilt, it all depends on how much you are willing to spend. Usually the cost is so much more than a used engine, that most people won't bother. This is true for a lot of things, but as they get more and more rare, rebuilding becomes your only option. I'm pretty sure that cylinder can be sleeved, but a lot depends on the damage. Melling makes centrifugally cast sleeves in various sizes. The machine shop will determine what size you will need. Finding good internal parts is getting to be an issue for those, so it is good to have some used engines to part out. Getting custom pistons and bearings made is not for the faint of wallet. And while sometimes substitutes can be found, the research it takes for that is not something for a single bike.