|Groco, Todd, Moeller, Ronco and Vetus all make replacement rigid tanks. I have also had considerable success with a Vetus flexible holding tank, which I used for 2 decades in my C&C 25. You have to properly secure it so that it won't shift around, but we never had any problems with odors or leakage. The advantage is that the flexible tanks come in various sizes and can be fit into odd-shaped lockers or corners. Western Marine is a Canadian distributor for Groco tanks and your local retailer can order from them. Jamestown Distributors (www.jamestowndistributors.com) displays a large selection of tanks from several manufacturers.
Be careful of changing the tank location as that will affect the balance of the boat, taking it away from the original design specs.
Marcus from Carriden
Mk III, Hull #847