Haven't heard of one of these snapping.
Is it possible that the bike was running with low oil for long periods of time? Unless the chain tensioner got messed up I don't see why it would ever be under enough pressure to cause it to snap. Can't rule out the possibility of simply a failed link. But even then, under normal riding conditions and properly lubricated... I would think if it was going to snap it would have done so a long time ago.
Anyway, follow this link to Ronayers: -
http://www.ronayers.com/Fiche/TypeID/26 ... odel/GZ250
They have just about everything that I can see. Gaskets are like $5 a piece and the whole cam chain package is only $80.