Replacing it is as easy as dumping boat bucks at it (301 of them):
Catalina Direct: Holding Tank For C-27 V Berth
There are other, suitable tanks available from Defender and the like that don't cost that much.
Actually removing the old and putting the new in depends largely on your skill set, your tool set, and/or the availability of yet another supply of boat bucks.
Make sure when reinstalling that it is held in place securely enough to hold it's own filled weight against the shock load of pounding into a head sea.
They do make glues for Poly tanks, you may be able to repair it.