that snapped kickstart boss is a common problem due to newbies stabbing at the kickstart, not letting it mesh with the gears first.
your right in saying that the halfs need to be together as they are machined as a pair, as are the head and cam cover!
xs360, 250 and 400 crancases are all the same, may be able to snag one cheap somewhere.
when you say clean barrels I take it as the finned outer, if you mean clean the bores, then honing is the way to go, you can do it yourself by hiring the honers and using a drill